Take two drugs and the effect may not be the sum of each alone. One can change how the other is absorbed, distributed, metabolized, or excreted. That change is a drug interaction, and it can make a drug stronger, weaker, or produce an entirely new side effect. Some interactions are mild. Others are dangerous.

The most common mechanism involves liver enzymes, especially the cytochrome P450 family. Some drugs inhibit these enzymes, slowing the breakdown of other drugs and raising their blood levels. Others induce them, speeding up metabolism and reducing the effect of a companion drug. Warfarin is a classic victim: antibiotics, NSAIDs, and even some foods can change its effect and shift a patient from therapeutic to dangerous. Interactions can also happen at the transporter level, in the stomach, or through additive effects on the same organ system.

Types of interactions:

• Pharmacokinetic. One drug changes the absorption, distribution, metabolism, or excretion of another.
• Pharmacodynamic. Two drugs act on the same system, adding or opposing effects.
• Food and drug. Grapefruit juice inhibits CYP3A4; leafy greens affect warfarin.
• Drug-disease. A drug can worsen an existing condition.

Pharmacists screen for interactions before dispensing. Patients should tell every prescriber about all medications, including supplements.