Recreational drugs include alcohol (as found in beer, wine, and distilled spirits); cannabis and hashish; nicotine (tobacco); caffeine (coffee and black tea); and the controlled substances listed as illegal drugs in the Single Convention on Narcotic Drugs (1961) and the Convention on Psychotropic Substances (1971) of the United Nations. What controlled substances are considered illegal drugs varies by country, but usually includes methamphetamines, heroin, cocaine, and club drugs. In 2009, it was estimated that about 3% to 6% of people aged 15 to 65 had used illegal drugs at least once (149 to 270 million).
Many researchers have explored the etiology of recreational drug use. Some of the most common theories are: genetics, personality type, psychological problems, self-medication, gender, age, instant gratification, basic human need, curiosity, rebelliousness, a sense of belonging to a group, family and attachment issues, history of trauma, failure at school or work, socioeconomic stressors, peer pressure, juvenile delinquency, availability, historical factors, or sociocultural influences. There has not been agreement around any one single cause. Instead, experts tend to apply the biopsychosocial model. Any number of these factors are likely to influence an individual’s drug use as they are not mutually exclusive. Regardless of genetics, mental health or traumatic experiences, social factors play a large role in exposure to and availability of certain types of drugs and patterns of drug use.
According to addiction researcher Martin A. Plant, many people go through a period of self-redefinition before initiating recreational drug use. They tend to view using drugs as part of a general lifestyle that involves belonging to a subculture that they associate with heightened status and the challenging of social norms. Plant says, “From the user's point of view there are many positive reasons to become part of the milieu of drug taking. The reasons for drug use appear to have as much to do with needs for friendship, pleasure and status as they do with unhappiness or poverty. Becoming a drug taker, to many people, is a positive affirmation rather than a negative experience.”
Anthropological research has suggested that humans "may have evolved to counter-exploit plant neurotoxins". The ability to use botanical chemicals to serve the function of endogenous neurotransmitters may have improved survival rates, conferring an evolutionary advantage. A typically restrictive prehistoric diet may have emphasised the apparent benefit of consuming psychoactive drugs, which had themselves evolved to imitate neurotransmitters. Chemical–ecological adaptations, and the genetics of hepatic enzymes, particularly cytochrome P450, have led researchers to propose that "humans have shared a co-evolutionary relationship with psychotropic plant substances that is millions of years old."
Severity and type of risks that come with recreational drug use vary widely with the drug in question and the amount being used. There are many factors in the environment and within the user that interact with each drug differently. Overall, some studies suggest that alcohol is one of the most dangerous of all recreational drugs; only heroin, crack cocaine, and methamphetamines are judged to be more harmful. However, studies which focus on a moderate level of alcohol consumption have concluded that there can be substantial health benefits from its use, such as decreased risk of cardiac disease, stroke and cognitive decline. This claim has been disputed. Researcher David Nutt stated that these studies showing benefits for "moderate" alcohol consumption lacked control for the variable of what the subjects were drinking, beforehand. Experts in the UK have suggested that some drugs that may be causing less harm, to fewer users (although they are also used less frequently in the first place), include cannabis, psilocybin mushrooms, LSD, and ecstasy. These drugs are not without their own particular risks.
The concept of "responsible drug use" is that a person can use drugs recreationally or otherwise with reduced or eliminated risk of negatively affecting other aspects of one's life or other people's lives. Advocates of this philosophy point to the many well-known artists and intellectuals who have used drugs, experimentally or otherwise, with few detrimental effects on their lives. Responsible drug use becomes problematic only when the use of the substance significantly interferes with the user's daily life.
Responsible drug use advocates that users should not take drugs at the same time as activities such as driving, swimming, operating machinery, or other activities that are unsafe without a sober state. Responsible drug use is emphasized as a primary prevention technique in harm-reduction drug policies. Harm-reduction policies were popularized in the late 1980s, although they began in the 1970s counter-culture, when cartoons explaining responsible drug use and the consequences of irresponsible drug use were distributed to users. Another issue is that the illegality of drugs in itself also causes social and economic consequences for those using them—the drugs may be "cut" with adulterants and the purity varies wildly, making overdoses more likely—and legalization of drug production and distribution would reduce these and other dangers of illegal drug use. Harm reduction seeks to minimize the harm that can occur through the use of various drugs, whether legal (e.g., alcohol and nicotine), or illegal (e.g., heroin and cocaine). For example, people who inject illicit drugs can minimize harm to both themselves and members of the community through proper injecting technique, using new needles and syringes each time, and proper disposal of all injecting equipment.