The effects of tolerance mean that increasingly higher doses of a drug are needed to maintain the same effect. The body learns to tolerate the drug in the system.

Alcohol, barbiturates, heroin and amphetamine are all drugs to which the body can build up tolerance.

People can develop tolerance to both illicit drugs and prescription medications. Tolerance is a physical effect of repeated use of a drug, not necessarily a sign of addiction. For example, patients with chronic pain frequently develop tolerance to some effects of prescription pain medications without developing an addiction to them.