Effects

Cannabis has psychoactive and physiological effects when consumed, usually by smoking or ingestion. The minimum amount of THC required to have a perceptible psychoactive effect is about 10 micrograms per kilogram of body weight (which, in practical terms, is a varying amount, dependent upon potency). A related compound, Δ9-tetrahydrocannabivarin, also known as THCV, is produced in appreciable amounts by certain drug strains. This cannabinoid has been described in the popular literature as having shorter-acting, flashier effects than THC, but recent studies suggest that it may actually inhibit the effects of THC. Relatively high levels of THCV are common in African dagga (marijuana), and in hashish from the northwest Himalayas.

Health issues
Main article: Health issues and the effects of cannabis
Cannabis use has been alleged to be associated with several illnesses. Due to its illegal status in many countries, it has been difficult to research these claims. Cannabis is rumoured to cause lung cancer, yet studies and tests have proven inconclusive. When mixed with tobacco however, there is a risk.
Cannabis use has been linked to psychosis by several peer-reviewed studies. A 1987 Swedish study claiming a link between cannabis use and schizophrenia was criticized for not differentiating between cannabis use and the use of other narcotics, and its results have not been verified by other studies. More recently, the Dunedin Multidisciplinary Health and Development Study published research showing an increased risk of psychosis for cannabis users with a certain genetic predisposition, held by 25% of the population. In 2007, a study published in The Lancet and a poll of mental health experts showed that a growing number of medical health practitioners are convinced that cannabis use increases susceptibility to mental illness, accounting for 14% of United Kingdom psychosis cases; however, the risk to an individual smoking cannabis is only increased by 2%.

Relationship with other drugs
Since its origin in the 1950s, the "gateway drug" hypothesis has been one of the central pillars of cannabis drug policy in the United States. The argument is that people, upon trying cannabis for the first time and not finding it dangerous, are then tempted to try other, harder drugs. This model of cause and effect has been debated. Some argue that the purported relationship between marijuana and more illicit drugs, as proposed by the "gateway theory," is methodologically flawed--it is a spurious association. A common argument is that a beginning user of cannabis, not finding it dangerous, will see the often stark difference between public information regarding the drug (much of which is misleading or fabricated, e.g. DARE), and come to distrust the sources of that information in general, and specifically regarding information on other drugs. Some studies support the "gateway drug" model. An example from 2007: A stratified, random sample of 1943 adolescents was recruited from secondary schools across Victoria, Australia, at age 14–15 years. This cohort was interviewed on eight occasions until the age of 24–25 years. At age 24 years, 12% of the sample had used amphetamines in the past year, with 1–2% using at least weekly. Young adult amphetamine use was predicted strongly by adolescent drug use and was associated robustly with other drug use and dependence in young adulthood. Associations were stronger for more frequent users. Among young adults who had not been using amphetamines at age 20 years, the strongest predictor of use at age 24 years was the use of other drugs, particularly cannabis, at 20 years. Those who were smoking cannabis at the age of 15 were as much as 15 times more likely to be using amphetamines in their early 20s.
Analysts have hypothesized that the illegal status of cannabis is a possible cause of a gateway drug effect, reasoning that cannabis users are likely to become acquainted with people who use and sell other illegal drugs in order to acquire cannabis. But it is said to be that Marijuana is not as harmful or addicting as any other drug. Some contend that by this argument, alcohol and tobacco may also be regarded as gateway drugs. Studies have shown that tobacco smoking is a better predictor of concurrent illicit hard drug use than smoking cannabis.

Comparison of addiction vs. physical harm for 20 drugs as estimated by an article in The Lancet
A current doctoral thesis from Karolinska Institutet, Stockholm, on the neurobiological effects of early life cannabis exposure, gives support for the cannabis gateway hypothesis in relation to adult opiate abuse. THC exposed rats showed increased motivation for opiate drug use under conditions of stress. However, the cannabis exposure did not correlate to amphetamine use.
A study published in The Lancet on 24 March 2007 was twenty drugs were assigned a risk from zero to three. Dr. David Nutt et al. asked medical, scientific and legal experts to rate 20 different drugs on nine parameters:
Physical harm (Acute, Chronic, and Intravenous harm)
Dependence (Intensity of pleasure, Psychological dependence, Physical dependence)
Social harms (Intoxication, Other social harms, Health-care costs)
Cannabis was ranked seventeenth of twenty for mean physical harm score and eleventh for mean dependence score. Not shown is the mean social harm score, which rated ninth, in a tie with Amphetamine.

Classification
While many drugs clearly fall into the category of either Stimulant, Depressant, Hallucinogen, or Antipsychotic, cannabis, containing both THC and CBD, exhibits a mix of all sections, leaning towards the Hallucinogen section due to THC being the primary constituent.