A drug may interact with another by inhibiting its action, potentiating its action (the sum is greater than the parts), or by simple summation (addition) of effects.
The interaction may take place:
Before absorption or administration – for example, antacids bind tetracycline in the gut and prevent absorption.
By interfering with protein binding – one drug may displace another from binding sites on plasma proteins. The action of the displaced drug will be increased because more drug is now available; for example, anticoagulants are displaced by analgesics.
During metabolism or excretion of the drug – some drugs increase or decrease the activity of liver enzymes which metabolise drugs, thus affecting their rate of destruction; for example, barbiturates, nicotine, and alcohol all activate hepatic enzymes. Altering the pH of urine will affect the excretion of drugs via the kidney.
At the drug receptor – one drug may displace another at the receptor, affecting its efficacy or duration of action.