Drug tolerance describes a user’s reduced reaction to a drug after its repeated use. An increase in dosage may re-amplify the drug’s effect; however, this may also accelerate tolerance, further reducing the drug’s effects. 

One may also develop tolerance to side effects; when this concerns non-addictive, prescribed medications, tolerance may be desirable. A medical intervention that has an objective to increase tolerance (for example, allergen immunotherapy, in which one is exposed to larger and larger amounts of allergen to decrease one’s allergic reactions) is called drug desensitization.

The opposite of drug tolerance is drug sensitization, in which case the user’s reaction increases following repeated use. High tolerance may sometimes lead to drug sensitization. For example, heavy drinkers initially develop a tolerance to alcohol, which requires them to drink larger amounts to achieve a similar effect. However, excessive drinking can then cause liver damage, which puts them at risk of lethal intoxication when drinking even very small amounts of alcohol.

Drug sensitization can also occur in users of stimulants such as cocaine or meth. What had been a recreational dose may become enough to cause psychosis in regular users, or users who previously had a psychotic episode may be more likely to have one in the future and at lower doses once usage continues.

In order for symptoms of withdrawal to occur, one must have first developed a physical or psychological drug dependence, or a combination of both. Dependence develops after consuming one or more substances over a sustained period of time.

Dependence produces withdrawal symptoms that vary with the type of drug in question. For example, withdrawal symptoms from opiates include anxiety, sweating, vomiting, and diarrhea. Alcohol withdrawal symptoms include irritability, fatigue, shaking and nausea. 

There are different stages of withdrawal as well; generally, a person will start to crash, progress to feeling worse, then hit a plateau before the symptoms begin to dissipate. Withdrawal from certain drugs like benzodiazepines can be fatal. While it is seldom fatal to the user, withdrawal from opiates can cause miscarriage, due to fetal withdrawal. 

The term “cold turkey” is used to describe sudden cessation of substance use and the physiologic manifestations that follow. The symptoms from withdrawal may be even more dramatic when the drug has concealed an extended period of malnutrition, disease, or sleep deprivation, common conditions often developed as a secondary consequence of drug addiction. These conditions may resurface when the drug is removed and be confused with withdrawal symptoms.

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