Why does cannabis potency matter?

29 June 2009 - Of the many people who use cannabis world wide, also known as marijuana, very few understand the increase in its potency over time. Cannabis has changed dramatically since the 1970s. New methods of production such as hydroponic cultivation have increased the potency and the negative effects of tetrahydrocannabinol (THC), the most psychoactive of chemical substances found in marijuana. It is important to understand cannabis potency because of its link to health problems including mental health.

The amount of THC in a cannabis sample is generally used as a measure of cannabis potency. One of the most comprehensive studies was conducted in 2004 by the European Monitoring Centre on Drugs and Drug Addiction (EMCDDA) and concluded that a modest increase in aggregate cannabis potency had occurred, possibly related to the use of intensive indoor cultivation methods. The authors noted that THC content nonetheless varied widely.

While the United Kingdom Home Office study in 2008 found little change: from a median potency of sinsemilla cannabis of 14% among samples in 2004/5 compared to 15% in 2008, long-term increases have been reported in the United States with an average potency of 10% in 2008.

Multiple methodological issues have been raised, which affect the capacity to generate comparable data and infer trends. Important variables to be considered include the phytochemistry; type of cannabis product; cultivation method; sampling; and stability.

As detailed below, each of these can affect the potency estimates.

Plant part used: The secretion of THC is most abundant in the flowering heads and surrounding leaves. The amount of resin secreted is influenced by environmental conditions during growth (light, temperature and humidity), sex of the plant, and time of harvest. The THC content varies between parts of the plant: from 10-12 % in flowers, 1-2 % in leaves, 0.1-0.3 % in stalks, to less than 0.03 % in the roots.

Product type: There are three main types of cannabis products: herb (marijuana), resin (hashish) and oil (hash oil). Cannabis herb comprises the dried and crushed flower-heads and surrounding leaves. It often contains up to 5% THC. Sinsemilla, derived from the unfertilized female plant, can be much more potent, however. Cannabis resin can contain up to 20% THC.

The most potent form of cannabis, however, is cannabis oil, derived from the concentrated

resin extract. It may contain more than 60% THC. The increase in market share of a particular product type can influence the reported average potency values. For example, the reported rise in the average THC content to 10% in seized samples in 2008 by the United States Office of National Drug Control Policy is associated with a market share of 40% for high potency cannabis (presumably indoor-grown).

Cultivation methods: The cannabis plant grows in a variety of climates. The amount and quality of resin produced depends on the temperature, humidity, light and soil acidity/

alkalinity. Outdoor-produced herbal cannabis, therefore, shows considerable variation in potency. Intensive indoor cultivation of female plants and clones, under artificial light,

often without soil (hydroponic cultivation), and optimised cultivation conditions, produces cannabis of a consistently higher potency.

Sampling: Most data on cannabis potency are derived from the analysis of seized samples. This means that these samples need to be representative of the entire seizure so that inferences and extrapolations can be made.

Stability: THC is converted to cannabinol on exposure to air and light. This process reduces the THC concentration, especially in old samples which have not been stored under suitable conditions (such as a dark, cool place). It is believed that claims of increases in potency of cannabis preparations confiscated over a period of 18 years in the United States10 may have been affected by the stability of THC in old samples.

Only through examining these factors can we have a more systematic, scientific and comparable assessment of cannabis potency between places and over time.