Interaction mechanisms

Last updated: Sunday, July 05, 2015

The four most important mechanisms by which medicines interact are outlined below:

1. Absorption

One drug decreases the absorption from the gut of a second drug. For example:

  • Antacids can decrease the absorption of many drugs (e.g. ciprofloxacin, iron, oxytetracycline).

2. Metabolism

One drug increases or decreases the metabolism of a second drug. For example:

  • Rifampicin enhances the metabolism of female sex hormones, making the contraceptive pill unreliable.
  • Allopurinol inhibits the metabolic destruction of a cytotoxic metabolite of azathioprine, so that this can accumulate to cause toxicity.


Azathioprine and allopurinol 
Courtesy of Giorgiogp2, Wikimedia Commons

3. Elimination

One drug increases or decreases the rate at which a second drug is removed from the body. For example:

  • Amiodarone reduces the elimination of digoxin by the kidney.
  • Cholestyramine enhances the excretion of the active metabolite of leflunomide.

4. Pharmacodynamic

Two drugs have additive or opposing pharmacological effects. For example:

  • NSAIDs cause fluid retention, which may at least partially counteract the action of diuretics.
  • Sedative drugs have additive CNS depressant effects with alcohol.