Cannabis (drug)
Cannabis, also known as marijuana (sometimes spelled 'marihuana') among many other names, refers to any number of preparations of the Cannabis plant intended for use as a psychoactive drug or for medicinal purposes. The word marijuana comes from the Mexican Spanish word, marihuana. According to the United Nations, cannabis 'is the most widely used illicit substance in the world.'
The typical herbal form of cannabis consists of the flowers and subtending leaves and stalks of mature pistillate of female plants. The resinous form of the drug is known as hashish (or merely as 'hash')
The major psychoactive chemical compound in cannabis is -tetrahydrocannabinol (commonly abbreviated as THC). Cannabis contains more than 400 different chemical compounds, including at least 66 other cannabinoids cannabidiol (CBD), cannabinol (CBN) and tetrahydrocannabivarin (THCV), etc.) which can result in different effects from those of THC alone.

Cannabis use has been found to have occurred as long ago as the 3rd millennium BC[8] In modern times, the drug has been used for recreational, religious or spiritual, and medicinal purposes. The UN estimated that in 2004 about 4% of the world's adult population (162 million people) use cannabis annually, and about 0.6% (22.5 million) use it on a daily basis. The possession, use, or sale of cannabis preparations containing psychoactive cannabinoids became illegal in most parts of the world in the early 20th century. Cannabishas psychoactive and physiological effects when consumed. The minimum amount of THC required to have a perceptible psychoactive effect is about 10 micrograms per kilogram of body weight.  Aside from a subjective change in perception and, most notably, mood, the most common short-term physical and neurological effects include increased heart rate, lowered blood pressure, impairment of short-term and working memory,  psychomotor coordination, and concentration. Long-term effects are less clear.
ClassificationWhile many drugs clearly fall into the category of either stimulant, depressant, or hallucinogen, cannabis exhibits a mix of all properties, perhaps leaning the most towards hallucinogenic or psychedelic properties, though with other effects quite pronounced as well. Though THC is typically considered the primary active component of the cannabis plant, various scientific studies have suggested that certain other cannabinoids like CBD may also play a significant role in its psychoactive effects.

Medical useCannabis used medically does have several well-documented beneficial effects. Among these are: the amelioration of nausea and vomiting, stimulation of hunger in chemotherapy and AIDS patients, lowered intraocular eye pressure (shown to be effective for treating glaucoma), as well as general analgesic effects (pain reliever).
Cannabis was manufactured and sold by U.S. pharmaceutical companies from the 1880s through the 1930s, but the lack of documented information on the frequency and effectiveness of its use makes it difficult to evaluateits medicinal value. In 1915, one medical supply house, the Frank S. Betz Co. of Hammond, Indiana, offered 'Cannabis Indica (Cannabis sative)' as one of about 70 'Crude Drugs' for $2.25 per lb., and offered a 10 percent discount for the purchase of 5 lbs. The same company advertised 'Tincture Cannabis Indica, U.S.P.,' for 80 cents per lb Cannabis in the form of a tincture and a fluid extract is documented in a 1929–30 Parke Davis & Co catalog, and is listed as an active ingredient in ten products for cough, colic, neuralgia, cholera mordus and other medical conditions, as well as a 'narcotic, analgesic, and sedative'. The catalog also lists compound medications containing cannabis that in some cases were apparently formulated by medical doctors, in its Pills and Tablets section.
As cannabis is further legalized for medicinal use, it is possible that some of the foregoing compound medicines, whose formulas have been copied exactly as published, may be scientifically tested to determine whether they are effective medications.
Less confirmed individual studies also have been conducted indicating cannabis to be beneficial to a gamut of conditions running from multiple sclerosis to depression. Synthesized cannabinoids are also sold as prescription drugs, including Marinol (dronabinol in the United States and Germany) and Cesamet (nabilone in Canada, Mexico, the United States and the United Kingdom).
Currently, the U.S. Food and Drug Administration (FDA) has not approved smoked marijuana for any condition ordisease in the United States, largely because good quality scientific evidence for its use from U.S. studies is lacking; however, a major barrier to acquiring the necessary evidence is the lack of federal funding for this kind of research. Regardless, fourteen states have legalized cannabis for medical use. Canada, Spain, The Netherlands and Austria have also legalized cannabis for medicinal use.
Long-term effects
The smoking of cannabis is the most harmful method of consumption, as the inhalation of smoke from organic materials can cause various health problems.
By comparison, studies on the vaporization of cannabis found that subjects were 'only 40% as likely to report respiratory symptoms as users who do not vaporize, even when age, sex, cigarette use, and amount of cannabis consumed are controlled. Another study found vaporizers to be 'a safe and effective cannabinoid delivery system

Systematic reviews of medical literature have found no evidence of a relationship between cannabis use and cancer. While a study in New Zealand of 79 lung-cancer patients suggested daily cannabis smokers have a 5.7 times higher risk of lung cancer than non-users, another study of 2252 people in Los Angeles failed to find a correlation between the smoking of cannabis and lung, head or neck cancers. Some studies have also found that moderate cannabis use may protect against head and neck cancers, as well as lung cancer. Some studies have shown that cannabidiol may also be useful in treating breast cancer. These effectshave been attributed to the well documented anti-tumoral properties of cannabinoids, specifically tetrahydrocannabinol (THC) and cannabidiol Cannabis use has been assessed by several studies to be correlated with the development of anxiety, psychosis, and depression. A 2007 meta-analysis estimated that cannabis use is statistically associated, in a dose-dependent manner, to an increased risk in the development of psychotic disorders, including schizophrenia. No causal mechanism has been proven, however, and the meaning of the correlation and its direction is a subject of debate that has not been resolved in the scientific community. Some studies assess that the causality is more likely to involve a path from cannabis use to psychotic symptoms rather than a path from psychotic symptoms to cannabis use, while other studies assess the opposite direction of the causality, or hold cannabis to only form parts of a 'causal constellation', while not inflicting mental health problems that would not have occurred in the absence of the cannabis use.
Though cannabis use has at times been associated with stroke, there is no firmly established link, and potential mechanisms are unknown. Similarly, there is no established relationship between cannabis use and heart disease, including exacerbation of cases of existing heart disease., Though some fMRI studies have shown changes in neurological function in long term heavy cannabis users, no long term behavioral effects after abstinence have been linked to these changes.