Abuse of cannabis resin in the form of marijuana, hashish or other preparations results in toxicity which varies widely depending on the amount of the psychoactive agent present. This in turn depends on ecological conditions, the part of the plant used, the preparation process and transport and storage conditions. In small doses cannabis acts as a mild euphoriant and sedative in large doses as a hallucinogen. Immediate effects can include : hilarity, loquacious euphoria; disinhibition; changes in perception of time and space; impaired co-ordination, judgement and memory; and increased visual and auditory sensitivity. Reliable evidence points to fragmentation of thought and an altered sense of identity, although clinical studies are often complicated by the intake of alcohol and other drugs by the cannabis users under observation. After high doses, illusion, delusions, depression, confusion, alienation and hallucinations may be experienced. Sometimes these may resemble a psychotic episode marked by fear and aggression. Pharmacological tolerance develops in time. There are no overt withdrawal symptoms, but users often show a psychological dependence.
Long-term use of marijuana impairs short-term memory and slows learning; it interferes with normal reproductive functions; adversely affects heart function; has serious effects on perception and skilled performance, such as driving and other complex tasks involving judgement or fine motor skills. Regular and prolonged use of cannabis may impair endocrine function, reduce immunity and lower resistance to infection. A marijuana cigarette contains more cancer-causing agents than the strongest tobacco cigarette. Long term use may give rise to conjunctivitis, bronchitis and chronic catarrhal laryngitis. It may heighten the possibility of cerebral atrophy. Marijuana is associated with multi-drug abuse, but no conclusive evidence exists to show that it leads on to abuse of stronger drugs such as LSD, heroin, cocaine. Marijuana has been adulterated with other drugs and sold on the street to unsuspecting users.
There are indications that show that people who regularly smoke large amounts of marijuana may experience changes in their brain chemistry that are identical to changes seen in the brains of people who abuse heroin, cocaine, amphetamines, nicotine and alcohol. There is strong support for the theory that all addictive drugs corrupt the same brain circuits, although to varying degrees.
The marijuana "joint" is smoked world-wide, and is the most commonly abused drug in the world. It has been estimated that one third of university students in the USA have tried marijuana; that one seventh are habitual users; and that there are 200 million users in the world. It is associated with a high incidence of psychosis in the Middle East. However, no conclusive evidence connects these conditions with cannabis abuse, nor with an increase in crime and violence rates.
According to a 1999 report, cannabis was by far the most widely used illicit drug in the EU; more than 40 million Europeans had tried it at least once. Nearly 40% of young adults in the UK, Denmark and Spain had already used it. Cannabis usage increased during the early 1990s, but had stabilized since then.
Regardless of the THC content, the amount of tar inhaled by marijuana smokers and the level of carbon monoxide absorbed are three to five times greater than among tobacco smokers. This may be due to the marijuana users' inhaling more deeply and holding the smoke in the lungs and because marijuana smoke is unfiltered.
A US study of college students has shown that critical skills related to attention, memory, and learning are impaired among people who use marijuana heavily, even after discontinuing its use for at least 24 hours. Researchers compared 65 "heavy users," who had smoked marijuana a median of 29 of the past 30 days, and 64 "light users," who had smoked a median of 1 of the past 30 days. After a closely monitored 19- to 24-hour period of abstinence from marijuana and other illicit drugs and alcohol, the undergraduates were given several standard tests measuring aspects of attention, memory, and learning. Compared to the light users, heavy marijuana users made more errors and had more difficulty sustaining attention, shifting attention to meet the demands of changes in the environment, and in registering, processing, and using information. These findings suggest that the greater impairment among heavy users is likely due to an alteration of brain activity produced by marijuana.
Marijuana remains the most commonly used illicit drug in the United States. There were an estimated 2.1 million people who started using marijuana in 1998. According to data from the 1998 National Household Survey on Drug Abuse (NHSDA), more than 72.0 million Americans (33 percent) 12 years of age and older have tried marijuana at least once in their lifetimes, and almost 18.7 million (8.6 percent) had used marijuana in the past year. In 1985, 56.5 million Americans (29.4 percent) had tried marijuana at least once in their lifetimes, and 26.1 million (13.6 percent) had used marijuana within the past year.
There is no scientific evidence that shows that smoked marijuana is useful or needed.
Marijuana has been demonstrated to overcome the side effects of chemotherapy (notably nausea and vomiting) and for relieving the symptoms of multiple sclerosis. Smoked marijuana lowers intraocular eye pressure (IOP) is subjects with normal IOP and patients with glaucoma. Clinical studies and survey data in healthy populations have shown a strong relationship between marijuana use and increased eating.