Science Cannabis

224 THE SCIENCE OF MARIJUANASociety in 1998, Dr. M. El Sohly from the University of Mississippi summarizedthe results obtained on more than 35,000 such samples since1980. Marijuana leaf samples (the type most common in United Statesseizures) had around 2% THC content in 1980 and most recently thefigures were 3.9% in 1996 and 4.1% in 1997. There was considerablefluctuation from year to year in the data — but it is clear that if there hasbeen any progressive increase in the potency of herbal marijuana it representsnot more than a doubling in THC content over nearly 20 years.The THC content in sensemilla (the female flowering heads) wasaround 6.5% in 1980, 9.2% in 1996, and 11.5% in 1997. Any increases inTHC content were attributed to improved culture conditions rather thanto any genetic improvements. Analysis of samples of cannabis resin orcannabis oil failed to show any discernible trends, with figures rangingfrom a 3% to a 19% THC content.In Britain the United Kingdom government's Forensic Science Serviceprovided data to the House of Lords (1998) enquiry on the THCcontent of cannabis samples seized in the United Kingdom (Fig. 6.4).They made the following statements:Cannabis resin, a wholly imported material, has a mean THC content of4%-5%, although the range is from less than 1% to around 10%. Thispattern has remained unchanged for many years.Herbal cannabis may be seen in a number of forms, but the materialmost commonly seized by Police and Customs in the UK has been importedin the form of compressed blocks; the mean THC content is also 4%-5% with a range similar to that of the resin.Until about eight years ago, "home grown" cannabis was a poor qualityproduct often grown in greenhouses or on windowsills and normally forpersonal use. However, the introduction of a number of horticultural techniqueshas lead to the widespread and large scale domestic indoor cultivationof cannabis with a much higher THC content. These techniques includehydroponics, artificial lighting, control of "day" length, heating andventilation, cloning of "mother plants", and perhaps most importantly, thedevelopment of plant varieties which produce higher THC levels. Themean THC content of so-called hydroponic cannabis is close to 10% witha range extending to over 20%.