No two people are alike, and no drug will affect every human body in exactly the same way

Some of the factors affecting drug action are:

  • Age
  • Size
  • Sex
  • Genetic inheritance
  • Physical/emotional condition


  1. Age
  • Standard dosages are based on the amount of a drug that will cause the desired effect in an average adult. The bodies of very young and very old patients don’t function exactly like the average adult body.
  • In infants, body systems are not fully developed, they may have trouble breaking down or excreting drugs
  • The body systems of the elderly may not function as efficiently as when they were younger – the aging process slows down the work of certain organs
  • Smaller doses or different drugs may be required when treating the very old or the very young


  1. Size
  • The size of the person and whether or not they are fat or thin have an effect on drug action
  • If an average dose of medication is given to a very tall or very obese or very small and thin patient, the concentration of the drug in the bloodstream will not be the right amount to produce the effect you want


  1. Sex
  • Women may react more strongly to certain drugs than men
  • They are generally smaller and have a higher proportion of fat than men


  1. Genetic Factors
  • The individual makeup of each person causes slight differences in processes like biotransformation
  • Some people have very unusual drug reactions that may be linked to genetic factors


  1. Disease Conditions
  • Diseases can strongly affect how patients respond to drugs
  • The organs necessary for biotransformation and excretion may be impaired
  • Diseases of the liver and kidneys can affect the processing and elimination of drugs
  • Any disease can change the effectiveness of a drug without warning


  1. Emotional Conditions
  • Mental state can be an important factor in the success or failure of drug therapy
  • Negative emotional states and strong feelings such as jealousy, anger, or fear may have a noticeable effect on drug action
  • A patients expectations can also affect the drug action, a psychological effect called the placebo effect can add to the effectiveness of medication therapy


Factors surrounding the administration of medications may also cause differences in people’s response to drugs:

Route:  drugs are absorbed, distributed, and excreted at different rates when given by different means or routes (drugs are quickest when injected into the bloodstream, slowest when administered by mouth)

Time of Day:  drugs that make a patient sleepy are ordered to be taken at bed-time; stomach-irritating drugs may be taken with meals to avoid discomfort.  Normal bodily functions also vary with the time of day, thus affecting drug action

Drug Taking History:  some drugs can collect in the body, so the dosage must be adjusted to avoid overmedicating, repeated doses of the same drug may make the patient less responsive to the drug.  Certain combinations of drugs can slow down or speed up the effects or can cause unusual or dangerous reactions.

Environmental Factors:  extremes of weather or temperature can affect the action of drugs because heat and cold influence body functions.  Heat relaxes the blood vessels and speeds up circulation so drugs act faster.  Cold slows their action by constricting the blood vessels and slowing circulation.  High altitude makes some drugs ineffective because of the lower levels of oxygen.


Norma Nicholson RN BA MA (Ed)

Youth Advocate, Educator, Public Speaker