The therapeutic effect:
• this is the drugs ability to produce a desired effect
• The pharmacological effects of drugs are generally similar between individuals, but there is considerable inter-individual variation in the intensity and duration of the effect. The reasons for this include:
1) Normal biological variation between individuals
2) Variations in sensitivity and responsiveness of mechanism of drug action
3) Body weight
4) Genetic and ethnic variations
5) Age
6) Nutritional factors
7) Intercurrent illness/co-morbidity
8) Drug interactions
• initial higher ‘loading dose’ can be used to reach therapeutic dose sooner
• ED50 (effective dose) – defined as the dose required to produce a defined therapeutic effect in 50% of the population. The dose may be effective for some individuals, but not for others.
Therapeutic index:
• a measure of drug safety (a high therapeutic index drug is safer than a low one)
• usually defined as LD50/ED50 (LD50 = dose that kills 50% of animals that receive it)
Tachyphylaxis – rapid diminution in the response to a drug; usually occurs in minutes or hours; usually due to the depletion of a second messenger
Tolerance – gradual decline in effectiveness of a drug; usually occurs over days or weeks; usually due to a change in receptor number (up or down regulation) or the development of an opposing physiological response
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