When two or more different drugs are taken together, an effect other than that achieved with each single drug may result. This is also true of drug side-effects. For example, taking two drugs that each cause peripheral neuropathy increases the likelihood of the occurrence of neuropathy.

It is not only prescribed medicines that can interact. The presence of food in the stomach can increase or decrease the absorption of drugs. There may also be interactions between prescribed medicines and recreational drugs or complimentary therapies, although these are often poorly researched.

Drug interactions are usually divided into four groups: antagonism, synergism, potentiation, and interaction with metabolism.

Antagonism means that one drug reduces or blocks the effect of another. There are various ways in which this can happen. For instance, drugs can interfere with each other's absorption in the gut, circulation in the blood, or uptake by cells.

Synergism means that two or more drugs work together against one target, producing an effect that is greater than the individual effect of the two drugs together (like combining two plus two and getting five). Synergistic interactions can be beneficial and treatments may be deliberately chosen for this effect. 

Potentiation means that drug A boosts the effects of drug B, often by increasing the levels of drug B in the blood. Like synergism, this may be useful in cases in which the beneficial effects of drug B are enhanced. However, the toxicities of drug B may also be potentiated, leading to an increased level of side-effects.