Their conclusions are that increased potency has been observed in some countries, but there is enormous variation between samples, meaning that cannabis users may be exposed to greater variation in the strength of the cannabis they use in a single year than over years or decades.

Cannabis samples tested in the United States, the Netherlands, the United Kingdom and Italy have shown increases in potency over the last decade, but no significant growth in other European countries or in New Zealand has been found during the same period.

THC is the active ingredient in cannabis, which produces the strongest psychoactive effect. In the United States, the level of THC in confiscated cannabis was 8.5% in 2006, up from 4.5% in 1997. Recent Dutch data show that the THC of cannabis sold in coffee shops more than doubled between 2000 and 2004, but has since levelled off.

THC content varies according to the part of the plant that is used, the method of storage, and cultivation techniques. Popular belief is that hydroponic or other methods of indoor cultivation produce higher concentrations of THC than occur naturally, but the jury is still out on this issue.

The ability to control the indoor environment means that plants can reach their full potential, which includes reaching the maximum level of THC. The increase in market share of indoor-grown cannabis seen in Australia as well as North America and Europe may have led to a more consistent product which could explain the potency increases reported in some countries.

While some public debate has linked large increases in cannabis potency to increased mental health problems, there are currently insufficient data to justify this claim, and care ought to be taken when considering policy decisions on this basis. Importantly, further research is required to understand whether cannabis users can, or do, alter their intake in response to a change in potency.

In their discussion of potential health risks, the authors point to studies that observe that some cannabis smokers, when faced with a 'strong' product, act rather like tobacco smokers and adjust their dose by increasing the interval between puffs, or holding smoke in their lungs for a shorter period of time. This behaviour may reduce possible harms caused by increased potency.

The authors also discuss the health risks of contaminants. Possible contaminants include naturally occurring ones such as fungi; growth enhancers and pesticides; and substances added for marketing purposes to 'bulk up' the weight. Lack of systematic monitoring for contaminants makes an assessment of risk difficult; it is important to learn more about the health risks of cannabis of ingesting contaminated cannabis -- for example, moulds are known to cause respiratory problems and lung disease.

The authors say "Given the relatively high prevalence of cannabis use it is important we have current, accurate information to help users make informed decisions about their use, and that policy development and media debate about the health harms associated with its use are guided by research evidence rather than rumour."

Note: If no author is given, the source is cited instead.

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