Cannabis preparations are primarily derived from the female plant of Cannabis sativa. The plant contains dozens of different cannabinoids (ElSohly, 2002; Iversen, 2007), but the primary psychoactive constituent in cannabis products is delta-9-tetrahydrocannabinol (THC) (Iversen, 2007; Pertwee, 2008). Administration of THC in pure form produces psychological and physical effects that are similar to those users report when they are smoking cannabis (Wachtel et al., 2002), and drugs that block the effects of THC on brain receptors also block the effects of cannabis in animals (Pertwee, 2008) and humans (Heustis et al., 2001). The effects of THC may also be modulated by cannabidiol (CBD), a nonpsychoactive compound that is found in varying amounts in most cannabis products (Iversen, 2007).
The THC content is at its highest in the flowering tops of the female cannabis plant. Marijuana (THC content in the range of 0.5% to 5%) comprises the dried flowering tops and leaves of the plant. Hashish (THC content in the range of 2% to 20%) consists of dried cannabis resin and the compressed flowers. Hash oil is an oil-based extract of hashish that contains between 15% and 50% THC (UNODC, 2006).
Some varieties of marijuana such as Sinsemilla (skunk) and "Nederwiet" ("Netherweed") may have THC content as high as 20% (EMCDDA, 2006).
Cannabis is usually smoked in a "joint", the size of a cigarette, or in a water pipe, with tobacco sometimes added to assist with burning. A typical joint contains between 0.25 and 0.75g of cannabis. The amount of THC delivered to the lungs varies between 20% and 70%, and 5% to 24% reaches the brain (Hall & Solowij, 1998; Heustis, 2005; Iversen, 2007). A dose of around 2 to 3 mg of bioavailable THC will produce a "high" in occasional users, who usually share a joint between multiple users. More regular users can use three to five joints of highly potent cannabis a day (Hall et al., 2001) Smokers typically inhale deeply and "hold" their breath to maximise absorption of THC. Marijuana and hashish may also be eaten, mixed in cakes or cookies (Wikipedia, 2008), or drunk in a liquid infusion (e.g. bhang lassi in India), but cannabis is most often smoked because this is the most efficient way to achieve the desired psychoactive effects (Iversen, 2007).
Because of uncertainties about the THC content of cannabis, "heavy" cannabis use is often defined as daily or near daily use (Hall & Pacula, 2003). Regular use over a period of years increases users' risks of experiencing adverse health and psychological effects (Hall & Pacula, 2003). Daily cannabis users are more likely to: be male, be less well educated, and regularly use alcohol, tobacco, amphetamines, hallucinogens, psychostimulants, sedatives and opioids (Hall & Pacula, 2003).
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