Drug tolerance occurs when a subject's reaction to a psychopharmaceutical drug (such as a painkiller, intoxicant, or antibacterial) decreases so that larger doses are required to achieve the same effect. Drug tolerance can involve both psychological and physiological factors. In addicted patients, the resulting pattern of uncontrolled escalating doses may lead to drug overdose.
Tolerance may be related to familiarity of "drug onset cues". For example, the mind and body can become conditioned in response to environmental cues such as the sight of a needle or an alcoholic beverage, and therefore can produce the foundation of physiological responses before an actual drug is introduced to the body.  Thus, if there is no drug that follows said perception, or, if the dose is too small to produce the expected effect, such can trigger intense cravings in the addict. Exemplified, this theory may explain why "just one drink", or even just the sight or presence of familiar alcohol cues can cause a relapse in the case of a recovering alcoholic.
Tachyphylaxis is a medical term referring to the rapid development of drug tolerance.
In a different context, drug tolerance can refer to the lenient policies of a government or organization toward drugs that are considered illegal in other areas. For example, the distribution of cannabis has been legal in the Netherlands since the early 1970s, and the state of California continues to test the legality of its medical marijuana laws.
Drug intolerance is another social issue. Many law enforcement and civic agencies (such as schools and scouting groups) advocate a zero tolerance policy towards drugs, meaning that any infraction of existing laws and regulations will be punished, no matter how small.
- Physiological tolerance
- Physical dependence
- Reverse tolerance