🧪 General Principles🟢 BasicPharmacokinetics & Pharmacodynamics

Drug Interactions

How one drug can change the effect of another when they are taken together, sometimes helpfully but often causing harm.

Key points

A drug interaction is when one drug changes the effect of another taken at the same time, which can increase or decrease the effect or cause harm.
Pharmacokinetic interactions change how much drug reaches its target, by altering absorption, distribution, metabolism or excretion.
For example, one drug may block the liver enzymes that break down another, so the second drug builds up to a harmful level.
Pharmacodynamic interactions happen at the target itself: two drugs with the same action add up, while drugs with opposite actions cancel out.
Some interactions are useful and deliberate, such as combining drugs that act at different steps to boost the overall effect.
Interactions matter most for drugs with a narrow safety margin, in older people, and in those taking many drugs at once.

Memory trick

Two routes to interact: pharmacokinetic (change the level, e.g. block metabolism) and pharmacodynamic (change the effect, add or cancel at the target); riskiest with narrow-margin drugs.

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🧪 General Principles🟢 BasicPharmacokinetics & Pharmacodynamics

Drug Interactions

How one drug can change the effect of another when they are taken together, sometimes helpfully but often causing harm.

Key points

A drug interaction is when one drug changes the effect of another taken at the same time, which can increase or decrease the effect or cause harm.
Pharmacokinetic interactions change how much drug reaches its target, by altering absorption, distribution, metabolism or excretion.
For example, one drug may block the liver enzymes that break down another, so the second drug builds up to a harmful level.
Pharmacodynamic interactions happen at the target itself: two drugs with the same action add up, while drugs with opposite actions cancel out.
Some interactions are useful and deliberate, such as combining drugs that act at different steps to boost the overall effect.
Interactions matter most for drugs with a narrow safety margin, in older people, and in those taking many drugs at once.

Memory trick

Two routes to interact: pharmacokinetic (change the level, e.g. block metabolism) and pharmacodynamic (change the effect, add or cancel at the target); riskiest with narrow-margin drugs.

All of Pharmacology More General Principles