Pharmacology
Medication is said to have a narrow or wide therapeutic index. This describes the ratio of desired effect to toxic effect. A medicine with a narrow therapeutic index (close to one) only does what people want it to do when the amount given is enough to put the organism in danger.  A medicine with a wide therapeutic index (greater than five) does what people want it to do and does not necessarily put the organism in danger. Medication with a narrow margin are more difficult to dose and give to a person, and may require therapeutic drug monitoring (examples are warfarin, some antiepileptics, aminoglycoside antibiotics). Most anti-cancer drugs have a narrow therapeutic margin; toxic side-effects are almost always encountered at doses needed to kill tumours.