The current anti-cannabis crusade in the UK press is going hot and heavy. I imagine we're all used to the "cannabis boy in drugs shame" tabloid headlines from over there, and, as I blogged a couple of days ago, we now see respectable newspapers like the Independent on Sunday flip-flopping on marijuana (now it's bad).
But sometimes, it's just too ridiculous. Here are the opening paragraphs of a story about potent weed from the Liverpool Echo
Police issue warning about super strength Cannabis
Mar 20 2007
by Ben Rossington, Liverpool Echo
SUPER-strength cannabis so potent that just one puff can cause schizophrenia is being grown by Merseyside drug gangs.
Cannabis resin, usually smuggled in from Morocco, has been replaced by home-grown super skunk as the drug of choice for sale by criminal gangs on Merseyside.
Experts warn this new strain of cannabis is so incredibly strong it can bring on the early signs of schizophrenia from a single puff.
Today Merseyside’s police chief has warned that organised gangs are moving into the production of the drug as a quick way of making cash.
Wow, that stuff must have a 150% THC content.
The article also repeats the claim that this super-skunk is 25 times more potent than what Brits are used to.
But here's what the most recent peer-reviewed scientific evaluation of THC levels in Europe
had to say:
Cannabis potency in Europe
There has been much recent interest in the possibility that the concentration of Δ9-tetrahydrocannabinol (THC), the active principle in cannabis, is now greater than it was. If pronouncements in the popular media are to be believed, the THC content (potency) is 10 or 20 times the levels of only a generation ago, giving apparent cause for concern about increased problems. The more cynical might comment that this is also a convenient rallying cry for those against an increasingly permissive attitude to cannabis use. But to what extent do the available data justify this fear? If one begins to explore the issue in more detail, it becomes evident that concerns about high potency cannabis are by no means new, and the reality appears both less alarming and more complex.
THE FACTS AS FAR AS THEY ARE KNOWN
A recent study  found that data overall were weak, but the evidence available suggested that the potencies of resin and herbal cannabis that have been imported into Europe have shown little or no change, at least over the past 10 years or so. This is hardly surprising, as these products have been made by traditional methods that have probably remained the same for generations. In Europe, average potencies of imported resin and herbal cannabis are typically between 2% and 8%. Cannabis (hash) oil is uncommon in Europe, but its THC content has also shown no clear trend over many years. What has changed throughout Europe and elsewhere is the appearance, from the early 1990s, of herbal cannabis grown from selected seeds by intensive indoor methods. This material, best described as domestically produced 'sinsemilla' (from the Spanish sin semilla—without seeds), is also known as 'skunk', 'buds' or 'nederwiet'. Its hydroponic cultivation, with artificial control of 'daylight' length, propagation of female cuttings and prevention of fertilization certainly does produce cannabis with a higher potency; on average it may be twice as high as imported herbal cannabis, although the two potency distributions overlap and some samples of imported cannabis are, and have always been, of high potency . The increased THC content of herbal cannabis produced by indoor methods is a consequence of both genetic and environmental factors as well as freshness (i.e. production sites are close to the consumer and storage degradation of THC is thus avoided). There is some evidence that the potency of domestically produced sinsemilla is gradually increasing, perhaps as a result of continual improvements in technique. This product is distributed through the same networks as other cannabis products but, as indicated by the presence of home-grow shops in some European countries, consumers are also producing the drug at home.
However, a note of caution is needed when assessing this information. Data on potency trends over 5 years or more were available only from five countries in Europe; in some of these the test sample sizes were low or unknown. Questions exist in terms of how representative the seizures are of the overall illicit market and in terms of the subsampling and selection of material from individual seizures for forensic testing. In addition, for a number of methodological reasons, both the reliability and comparability of data from different forensic laboratories were questionable. By far, the greatest number of THC analyses was carried out in Germany, with over 7000 measurements annually, but no distinction was made between imported and home-grown herbal cannabis. There has also been a rise in overall potency in North America, but in Australia and New Zealand the picture is less clear.
THE IMPLICATIONS FOR PUBLIC HEALTH
If the strength of some forms of cannabis has increased, then is this a cause for concern? The first matter to address is whether the availability of a more 'concentrated' form of a drug is in itself an issue. A parallel might be drawn here with the consumption of alcohol. Public health consequences of alcohol consumption are not a simple function of the strength of the beverage consumed, be it beer, wine or spirits. Rather, at population level, research suggests that it is the total quantity of alcohol consumed that is important rather than the concentrations in which it is sold. How far this parallel holds for cannabis is unknown, but it does raise the question of whether the availability of high potency cannabis impacts on total consumption levels of THC. It is still unknown whether those who smoke higher potency cannabis have higher blood levels of THC or whether they titrate the dose according to the subjective and relatively immediate pharmacological effects. It should be noted that even if we consider only the smoking of cannabis cigarettes/joints, all the following factors will influence an individual smoker's dose exposure: the amount used per cigarette/joint, sharing with others, the number of cigarettes/joints consumed per session, the number of sessions in any given time period, and individual smoking technique. As Hall et al.  note, age of onset of use and frequency of use are likely to be more influential than changes in potency in determining consumption levels.
It is also important to note that, as far as we can tell, for most countries the market share of sinsemilla appears to be currently quite low. For example, in the United Kingdom it is estimated that resin comprises 70% of consumption. Of the remainder, about half comprises 'traditional' herbal cannabis and half sinsemilla. In other words, if the effective potency (the weighted average) had been 5%, then the appearance of sinsemilla can be estimated to have increased this to no more than 6%.
There is more from this academic review at the link above. This week, I'll be talking to people in Britain about all this for a feature article out Friday.