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The opioid analgesics can be divided into agonists, agonist-
antagonists, and antagonists on the basis of their interaction with
opioid receptors (table 4).

Pure agonists
Pure µ-agonists are generally preferred over agonist-antagonist
drugs for management of moderate to severe pain. With no
ceiling effect for analgesia and the availability of multiple formula-
tions (table 5), they offer great flexibility to prescribers. Clinical
experience with these medications throughout the ages for
treatment of acute and chronic pain is extensive.

Clinical pharmacology
Although there is great intraindividual variation in the response to
the different pure µ-agonists, the pharmacodynamic profile is
similar across them. For analgesia, there is a concentration-
response relationship that continues to slope upward until the
patient becomes unconscious. Side effects are very common,
and the clinical challenge is to identify a dose associated with a
favorable balance between analgesia and side effects.
   The concept of relative potency has important implications for
the clinical use of opioid analgesics. All the opioids differ in
potency, which is defined as the dose required to generate a
given effect. If the doses of 2 opioids are appropriately adjusted,
the same level of effect should be obtainable. In this context,
therefore, potency does not mean strength of effect or efficacy.
The efficacy of 2 opioids, one more potent than the other, is the
same if the doses used are equianalgesic.
   Numerous controlled trials have been done in populations with
relatively little opioid exposure, to calculate the relative potencies
between different opioids and between the same opioids given
by different routes of administration. These studies have allowed
the construction of an equianalgesic dose table (see table 5). The
table describes relative potencies by listing the doses of different
drugs and the administration routes that are equianalgesic to a
standard, usually 10 mg of morphine given intravenously or
intramuscularly. The equianalgesic dose table represents the best
science but was developed from studies in selected populations.
                                            OVERVIEW OF CLINICAL PHARMACOLOGY / 17

Guidelines for switching opiods and routes of administration
have been developed and are based on use of the table as a
starting point for dose selection (see chapter 5, page 40).
   Adverse effects. The most important potential adverse effect
from use of the pure agonists is respiratory depression. These
                                                                         Continued on page 20

 Table 4. Classification of opioid analgesics for pain management in the
 United States

  Opioid type               Medications               Notes about therapy
  Pure agonists             Codeine                   • No clinically relevant ceiling
                            Dihydrocodeine              effect to analgesia; as dose is
                            Fentanyl                    raised, analgesic effects increase
                            Hydrocodone                 until analgesia is achieved
                            Hydromorphone               or dose-limiting side effects
                            Levorphanol                 supervene
                            Meperidine                • Most commonly used for
                            Methadone                   moderate to severe pain
  Agonist-antagonists       Partial agonists          • µ-Agonist with lower intrinsic
                            Buprenorphine               efficacy (partial agonists) or
                                                        agents that produce agonist
                            Mixed agonist-              effects at one receptor and
                            antagonists                 antagonist effects at another
                            Butorphanol                 (mixed agonist-antagonists)
                            Dezocine                  • Ceiling effect for analgesia
                            Nalbuphine                • Some produce psychotomimetic
                            Pentazocine                 side effects more readily than do
                                                        pure agonist opioids
                                                      • Potential to induce acute absti-
                                                        nence in patients with physical
                                                        dependency to agonist opioids
                                                      • In general, less preferred by
                                                        patients with opioid addiction
  Pure antagonists          Alvimopan*                • Compete with endogenous and
                            Methylnaltrexone*           exogenous opioids at µ receptor
                            Naloxone                    sites
                            Naltrexone                • Administered for prevention or
                                                        reversal of opioid effects
  Other                     Tramadol                  • µ-Agonist distinguished by a
                                                        mechanism of action that
                                                        includes effects on monoamines,
                                                        such as serotonin
* Not yet commercially available; minimal systemic absorption by enteral route.

 Table 5. Pure µ-agonists used for pain in the United States

  Opioid                  Equianalgesic          Half-life        Peak               Duration
  analgesic               doses*† (mg)           (hr)             effect (hr)        (hr)
  Morphine                10 IM/IV/SQ            2-3              0.5-1              3-4
                          20-30 PO‡              2-3              1-2                3-6

  Controlled-             20-30 PO‡              2-3              NA                 8-12
  release morphine
  Sustained-              20-30 PO‡              2-3              NA                 12-24
  release morphine
  Hydromorphone           1.5 IM/IV/SQ           2-3              0.5-1              3-4
                          7.5 PO                 2-3              1-2                3-6

  Oxycodone               20-30 PO               2-3              1-2                3-6

  Controlled-       20-30 PO                     NA               3-4                8-12
  release oxycodone
  Oxymorphone             1 IM/IV/SQ             NA               0.5-1              3-6
                          10 PR                  NA               1.5-3              4-6
                          15 PO
  Levorphanol             2 IM/IV/SQ             12-15            0.5-1              3-6
                          4 PO                   12-15            1-2                3-6
  Methadone               Variable               12-150           1-2                6-8

  Hydrocodone             30 PO                  2-4              1-2                3-6

  Fentanyl                50-100 µg              7-12             <10 min            1-2
  Fentanyl                NA                     NA               12-24              48-72
  transdermal system                                                                 per patch

  Oral transmucosal       NA                     7-12             15-30 min          1-2
  fentanyl citrate

 FDA, US Food and Drug Administration; IM, intramuscular; IV, intravenous; NA, not applicable;
 PO, by mouth; PR, per rectum; SQ, subcutaneous.

* Dose provides analgesia equivalent to 10 mg of morphine given by IM route. These ratios
  are useful guides when switching drugs or routes of administration. In clinical practice, the
  potency of the IM route is considered to be identical to IV and SQ routes.
† When switching from one opioid to another, incomplete cross-tolerance requires a reduction
  in the dose of the new drug by 25% to 50%, to prevent excessive opioid effects. Provision of
  “rescue” medication during the conversion period (a few days) prevents breakthrough pain
                                            OVERVIEW OF CLINICAL PHARMACOLOGY / 19

Toxicity                           Comments
Constipation, nausea,              Standard for comparison of opioids;
sedation are most common;          multiple routes available
respiratory depression is
rare when titrated to effect

                                   Once-a-day morphine recently approved in
                                   United States
Typical opioid effects             Potency and high solubility may be beneficial for
                                   patients requiring high opioid doses and for SQ
Typical opioid effects             Available as single entity or combined with aspirin
                                   or acetaminophen
Typical opioid effects

Typical opioid effects             Oral immediate release and extended release
                                   formulations are currently under FDA review

Typical opioid effects             With long half-life, accumulation possible after
                                   beginning or increasing dose
Typical opioid effects             Highly variable half-life and potential for
                                   accumulation require increased vigilance for
                                   development of opioid toxicity; can prolong QTc
Typical opioid effects             Available only in combination with acetaminophen
                                   or aspirin
Typical opioid effects             Can be administered as continuous IV or SQ
Typical opioid effects             Refer to package insert for equianalgesic dosing
                                   guidelines for oral and parenteral medication;
                                   currently available doses not usually recommended
                                   for opioid-naïve patients; not recommended for
                                   acute pain
Typical opioid effects             Not recommended for opioid-naïve patients;
                                   recommended starting dose for breakthrough
                                   pain is 200-400 µg, even with high “baseline” opioid

  that might result from relative underdosing. When switching to methadone from another
  drug, the reduction in the equianalgesic dose should be greater, usually 75% to 90%.
‡ Extensive survey data suggest that the relative potency ratio of IM to PO morphine, which
  has been shown to be 1:6 in an acute dosing study, is 1:2 to 1:3 with chronic dosing.

opioids produce a concentration-dependent shift in the carbon
dioxide response curve. At clinically appropriate doses, compen-
sation for the shift occurs and respiratory rate typically does not
decline. Tolerance to the respiratory effects usually develops
quickly, and doses can be steadily increased without risk. If
some other cardiopulmonary insult occurs, however, the patient’s
response may be greater than it would have been without the
opioid present.
   Clinical evidence of this phenomenon is observed when
patients receiving long-term therapy experience respiratory
compromise associated with a new insult, such as pneumonia,
and show improvement after administration of naloxone. The
response to the opioid antagonist in this situation does not
mean that the opioid was the primary driver for the respiratory
problem, but it does show that there is some ongoing effect
on respiratory reserve even after opioid therapy has continued
for a time.
   Other side effects more commonly have clinical impact (see
chapter 6, page 53). Nausea and mental clouding or sedation are
common, but tolerance to these effects usually develops within
days to weeks. Constipation is also very common, and adapta-
tion to this effect occurs much less reliably. Many patients
require ongoing laxative therapy during long-term treatment.
   Some patients experience fatigue, confusion, or other
psychotomimetic effects (such as nightmares or hallucinations),
myoclonus, other gastrointestinal effects (such as bloating,
symptoms of reflux, or anorexia), dysphoria or other mood
effects (such as mood lability), headache, urinary retention, or
sexual dysfunction. Itch is relatively common during acute
administration and rarely reflects a true allergic response. Many
factors may predispose to adverse effects, including advanced
age, medical comorbidities, and concurrent administration of
other drugs. Successful management of side effects increases
the likelihood of a favorable outcome and potentially allows the
use of a more efficacious opioid dose over time.
   Outcomes generally considered under the rubric of chemical
dependency or drug abuse should also be considered potential
adverse effects of opioid use. All opioids that have agonist
effects interact with deep brain structures that subserve “rein-
forcement and reward” mechanisms. It has been estimated that
at least 5% to 10% of people have variants of this system that
predispose to addiction to opioids or other drugs with potentially
reinforcing effects (see chapter 9, page 67). Presumably, these
individuals represent a group that is more likely to experience
euphoric effects when an opioid is first taken. The likelihood of
                                OVERVIEW OF CLINICAL PHARMACOLOGY / 21

addiction is thought to increase if this biologic predisposition
occurs in tandem with a complex and poorly understood set of
psychologic, social, and situational factors.
   In most patients, the disease of addiction presents at an
early age. A patient who has reached middle age without
developing compulsive use behaviors to potentially abusable
drugs, including alcohol and nicotine, appears to be at very low
risk. This is particularly true if there is also no family history of
addiction. Patients who may be at relatively increased risk must
be identified so that opioid administration can be structured in a
manner that lessens the liability.
   Development of true addiction is not the only concern during
long-term opioid administration and it indeed appears to be far
less common than problems related to misuse and abuse. Drug
diversion, a criminal act, also is rarely encountered but must be
considered among the risks of therapy. To date, there are very
few empirical data to help define the patterns of these behaviors
or their clinical meaning. However, acute short-term administra-
tion is clearly less likely than long-term administration to be
associated with any of these potential outcomes. Healthcare
providers, patients, and families require reassurance about these
concerns, but at the same time, prescribers must be aware of
the need for careful monitoring for nontherapeutic outcomes.

Members of the drug class
   Morphine. Morphine is often considered the prototype pure
µ-agonist. It is available in multiple formulations and has been
extensively used in management of both acute and chronic pain.
   Morphine has 2 biologically active metabolites, morphine-6-
glucuronide and morphine-3-glucuronide. Morphine-6-glucuronide
binds to the opioid receptor and is believed to contribute to the
effects of the parent compound. Morphine-3-glucuronide does
not bind to the receptor and is believed to contribute in some
cases to adverse effects such as myoclonus and confusion.
Usually, the metabolites are considered a clinical issue only when
their concentrations in the blood are likely to fluctuate differently
than the concentration of the parent compound. This can occur
during renal insufficiency, in which concentrations of the renally
cleared metabolites relative to the parent compound can become
very high. Patients with fluctuating renal insufficiency are, on
theoretical grounds, the most likely patients to be at risk for
unpredictable morphine effects because of a changing ratio
between metabolite and parent compound.
   Morphine is available in immediate-release and modified-
release formulations. The latter formulations have an 8- to

24-hour duration of effect, depending on the specific drug and
individual variation.
   Hydromorphone. Hydromorphone is significantly more potent
than morphine, permitting smaller volumes to be used when
injecting equianalgesic doses. Like morphine, it can be adminis-
tered through oral, parenteral (subcutaneous, intramuscular, and
intravenous), rectal, or intraspinal (epidural and intrathecal)
routes. Its relatively short half-life of elimination (2 to 3 hours)
facilitates dose titration but complicates efforts to use hydromor-
phone for chronic pain. Modified-release formulations, which will
increase the convenience of oral therapy for chronic pain, are in
development and are currently being reviewed by the US Food
and Drug Administration (FDA).
   Because hydromorphone is very soluble in water, high-
concentration solutions can be made and are particularly
suitable for subcutaneous administration, including continuous
subcutaneous infusion. A high-potency preparation (10 mg/mL)
is commercially available. Side effects associated with hydro-
morphone are qualitatively similar to those associated with
opioids in general, and most often include constipation, nausea,
and sedation. Hydromorphone may be preferred over morphine
for patients with decreased renal clearance, to preempt the
potential for toxicity from morphine metabolite accumulation.
   Oxycodone. Oxycodone is available in both an immediate-
release and a modified-release (8- to 12-hour duration) prepara-
tion. The immediate-release formulation is available as a single
entity and in combination with acetaminophen or aspirin. Lower
doses of oxycodone (eg, 2.5 mg, 5 mg, 7.5 mg, 10 mg) in com-
bination with a nonopioid coanalgesic are frequently used for
management of acute pain in patients with limited prior opioid
exposure. When these drugs are used, care must be taken not
to exceed the recommended maximal dose of the coanalgesic
(for example, 4 g or less of acetaminophen per day). The
modified-release formulation of oxycodone is now widely used
for management of chronic pain.
   Oxymorphone. Oxymorphone has a short half-life and is
both a potent congener of morphine and an active metabolite of
oxycodone. It is presently available in suppository and injectable
forms. Although an oral form is not yet available, immediate-
release and modified-release formulations are in development
and are currently under review by the FDA. Oxymorphone may
have particular utility for patients subject to drug-drug interac-
tions since it does not affect the CYP2D6 or CYP3A4 enzymes.
   Meperidine. Meperidine is not preferred for long-term use
because of the risk of toxicity associated with accumulation of
                               OVERVIEW OF CLINICAL PHARMACOLOGY / 23

the metabolite normeperidine. Normeperidine can cause dyspho-
ria, tremulousness, hyperreflexia, and seizures. It is renally
cleared, and use of meperidine in patients with kidney disease
is not recommended.
   Methadone. In the United States, methadone is commercially
available as a racemic mixture. The l-isomer is the opioid com-
pound; the d-isomer does not bind to the opioid receptor but
instead blocks the N-methyl-D-aspartate (NMDA) receptor. This
pharmacology has been adduced to explain methadone’s appar-
ent increased potency when it is administered to a patient who
is already receiving another opioid. There are anecdotal observa-
tions suggesting particularly good efficacy against some pains
that were otherwise poorly responsive to opioids.
   The unique pharmacology of methadone, its potential efficacy,
and its low cost have combined to increase interest in the drug.
This is appropriate as long as the challenges inherent in dosing a
medication with an uncertain potency and a long and variable
half-life (from 12 to more than 150 hours, with the usual half-life
approximating 24 hours) are appreciated. It is recommended that
a switch to methadone be accompanied by a large (75% to
90%) decrease in the calculated equianalgesic dose, to account
for the potential for high potency. Because the plasma concen-
tration of methadone rises to steady-state levels over 4 to 5 half-
lives, rapid titration to an effective dose can subsequently be
followed by continued escalation of the plasma concentration,
ultimately leading to toxicity (known as accumulation).
   Finally, there are recent reports linking methadone to prolon-
gation of the QTc interval. At a critical point, this prolongation
can predispose to life-threatening cardiac arrhythmia. In short,
methadone dosing requires close monitoring, use of low starting
doses, an adequate interval between dose changes, and caution
in patients who have heart disease or medications with concur-
rent effects on the QTc interval.
   Levorphanol. Levorphanol is another opioid with a long
half-life (usually 12 to 15 hours). It generally can be administered
at an interval of 6 hours and may be useful particularly in patients
who are unable to tolerate, or access, modified-release opioids.
   Codeine. Codeine is the most commonly used opioid for mild
or moderate acute pain. It is typically used in combination with
aspirin or acetaminophen. Clinical experience suggests that
nausea and constipation are more commonly encountered with
codeine than with equianalgesic doses of other opioids.
   Propoxyphene. Propoxyphene is a weak opioid agonist that,
when administered at typical doses, has an efficacy similar to
that of aspirin or acetaminophen. Like meperidine, propoxyphene

has an excitotoxic metabolite that can accumulate, particularly
in the setting of renal insufficiency. It is not preferred for
management of chronic pain or for use in older patients.
    Hydrocodone, dihydrocodeine. The oral analgesic potency
of hydrocodone and dihydro-codeine is approximately 50% to
100% that of oral morphine. In the United States, they are
available only in combination with acetaminophen or aspirin.
The doses provided in these combination products are such
that these medications typically are used for treatment of acute
moderate to severe pain in patients with limited opioid exposure.
    Fentanyl. Fentanyl is a synthetic opioid that is characterized
by both high potency and comparatively high lipid solubility. A
transdermal fentanyl patch is available for continuous opioid
analgesia, and an oral transmucosal formulation is available for
relief of brief, episodic severe pain (eg, breakthrough pain). Each
transdermal fentanyl patch provides 48 to 72 hours of pain relief
at steady state. Some patients consider the patch delivery form
and the long dosing interval to be favorable characteristics. The
fentanyl patch is particularly useful for patients who are unable to
swallow or absorb an orally administered opioid, and studies that
suggest a lesser potential for constipation provide support for a
trial treatment with fentanyl when this symptom is especially
    The pharmacokinetics of the transdermal system are complex
and may be variable across patients. The formulation produces a
subcutaneous depot, resulting in a slow onset of effect after a
dose change and in a prolonged apparent elimination half-life
(usually 24 hours) after the patch is removed. Steady-state con-
centrations are not approached for 1 to 3 days and sometimes
longer. Oral transmucosal fentanyl is approved for treatment of
cancer-related breakthrough pain but has been used for other
types of episodic severe pains in opioid-tolerant patients. This
formulation incorporates fentanyl into a lozenge that is sucked,
allowing partial absorption through the buccal mucosa. The
formulation has been shown to be effective and well tolerated
and has an onset of effect faster than comparable doses of
“immediate-release” oral opioids.

Use of agonist-antagonists for persistent pain generally is not
preferred because of their ceiling dose for analgesia and the
potential for inducing an acute abstinence syndrome in patients
taking opioid agonists. Some of these medications, such as pen-
tazocine and butorphanol, also have a higher likelihood of psy-
chotomimetic side effects than the pure agonists. Studies sug-
                               OVERVIEW OF CLINICAL PHARMACOLOGY / 25

gest that these opioids have a lower abuse potential than the
pure opioid agonists in the known addict population, but this
property has limited relevance in the general patient population.
Buprenorphine, a partial agonist, is now available in the
United States for office-based substitution treatment of
opioid addiction.

Pure antagonists
Opioid antagonists exert their pharmacologic effect by compet-
ing with endogenous and exogenous opioids at µ receptor sites.
Their role in pain management is primarily to prevent or reverse
opioid-induced adverse effects. Low doses of oral naloxone
have been shown to reverse opioid-induced bowel dysmotility
without reversing analgesia. Use of naloxone, however, is not
without risk, because some patients experience uncomfortable
signs of systemic opioid withdrawal. Methylnaltrexone and
alvimopan, opioid antagonists whose activity is restricted to
peripheral receptors when ingested orally, are currently
undergoing investigation for prevention or reversal of opioid-
induced bowel effects without reversal of analgesia or precipita-
tion of withdrawal symptoms. Some studies suggest that
ultra-low doses of opioid antagonist drugs have analgesic
effects (see chapter 2, page 9). The clinical utility of this
observation is currently under study.

Drug metabolism and potential interactions
Most opioids are metabolized through the liver microsomal
cytochrome P-450 (CYP) system. The enzymes CYP2D6 and
CYP3A4, which are responsible for metabolism of a wide variety
of drugs, are the most important enzymes for opioid metabolism.
Patients may lack normal levels of enzymatic activity to metabo-
lize opioids at expected rates because of genetic factors, severe
liver disease, or competition with other medications.
    The potential importance of enzyme activity is illustrated by
codeine. Codeine is actually a prodrug that requires metabolic
transformation to morphine. Patients who are slow metabolizers
at the CYP2D6 isozyme produce little morphine from codeine
and are unlikely to benefit after codeine administration.
    Concomitant treatment with an inducer of a particular enzyme
may lead to decreased levels of medications that are metabo-
lized by that enzyme; treatment with an inhibitor may lead to
increased levels. There are many potential interactions (table 6).
It is likely that these interactions can produce clinically relevant
effects, warranting closer monitoring, but further research is
necessary to clarify their clinical implications.

 Table 6. Potential drug interactions for major cytochrome P-450 enzymes
 CYP2D6 and CYP3A4

  Enzyme    Substrates                   Inhibitors           Inducers
  CYP2D6    Amitriptyline, buproprion,   Citalopram (weak),   Carbamazepine,
            clomipramine,                desipramine,         phenobarbital,
            clozapine, clonazepam,       fluoxetine,          phenytoin
            codeine, clonazepam,         olanzapine (weak),
            codeine, desipramine,        paroxetine,
            dextromethorphan,            sertraline,
            doxepin, fluoxetine,         venlafaxine (weak)
            haloperidol, hydrocodone,
            imipramine, methadone,
            modafinil, morphine,
            nortriptyline, olanzapine,
            oxycodone, paroxetine,
            sertraline, tiagabine,
            tramadol, venlafaxine
  CYP3A4    Alfentanil, alprazolam,      Dexamethasone,       Carbamazepine,
            amitriptyline,               dextromethorphan,    dexamethasone,
            buproprion, citalopram,      fluoxetine,          erythromycin,
            clozapine, cyclosporin,      paroxetine (weak),   modafinil,
            dexamethasone,               sertraline,          phenobarbital,
            dextromethorphan,            venlafaxine          phenytoin
            etoposide, fentanyl,
            fluoxetine, ifosfamide,
            imipramine, ketamine,
            lidocaine, meperidine,
            modafinil, paclitaxel,
            prednisone, sertraline,
            tamoxifen, tiagabine,
            venlafaxine, vincristine

There is an ever-enlarging pharmacopeia of opioid analgesics in
a variety of formulations and delivery systems. A fundamental
understanding of the clinical pharmacology of opioids can inform
drug selection and assist in anticipating, and managing, both
favorable and adverse opioid effects.
                                        OVERVIEW OF CLINICAL PHARMACOLOGY / 27

Suggested readings

Gammaitoni AR, Fine P, Alvarez N, et al. Clinical application of opioid equianal-
gesic data. Clin J Pain 2003;19:286-97

Gazelle G, Fine PG. Methadone for the treatment of pain. J Palliat Med 2003;


Liu M, Wittbrodt E. Low-dose oral naloxone reverses opioid-induced constipation
and analgesia. J Pain Symptom Manage 2002;23:48-53

Portenoy RK, Payne R, Coluzzi P, et al. Oral transmucosal fentanyl citrate (OTFC)
titration study. Pain 1999;79:303-12

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