A wide variety of exogenous substances have addictive properties. Naturally occurring substances such as tobacco, cannabis and alcohol have been used by humankind for thousands of years. Synthetic compounds such as heroin, cocaine and benzodiazepines have become available in the last few centuries. For a substance to be addictive it must usually provide some psychological reward, example relief of anxiety, and be capable of inducing physical dependence. Substance addicts often crave the substance when it is not available because of psychological and physical withdrawal effects. Addicts often act in ways that damage themselves or others in order to get and use the desired substance. Repeated exposure to the same dose may result in a diminishing response. This is known as tolerance, where the addict feels that they need to up the dose to achieve the same effect as before. Dependence can be physical or psychological. Abusers of alcohol may drink in order to cope with stressful situations.