But there is little science behind such claims, even though 'hydro' now appears to be the most common way of growing cannabis, according to a report by Australia's National Crime Authority.

"This is an area with a plethora of speculation in the absence of evidence," says Wayne Hall, former director of Australia's National Drug and Alcohol Research Centre and now a professor at the University of Queensland. Given so much public concern over hydroponic marijuana, Hall wondered why no studies have been funded in Australia, and there are "no plans to do any," he said. Instead, some state governments are proposing that penalties for growing hydroponic cannabis be raised, even though there's no data on whether it is more potent or has higher THC content.

'Hydro' refers to cannabis that is grown indoors in a soil-free environment. Variables like nutrients, temperature and the number of hours' daylight are precisely controlled to ensure optimum flowering – the flowers being the part of the plant that is richest in THC. No male plants are permitted – a denial that sends the females into a flowering frenzy of growth: "Nature's highest expression of unrequited female botanical passion," as physician Ethan Russo at the University of Montana once observed.

This article is a boxout that goes with our feature – Marijuana: What science has to say, read the full article here.

These techniques are designed to produce higher potency, but it is not always guaranteed. A 2004 report commissioned by the European Monitoring Centre on Drugs and Drug Addiction (EMCDDA), titled "An overview of cannabis potency in Europe", found that THC levels of hydroponic cannabis could be two to three times the level of imported varieties. But they also noted a tremendous range of THC levels measured in imported varieties over the last 20 years. "It is clear cannabis users have constantly been exposed in almost random fashion to unexpectedly high and low amounts of THC," the report noted.

Given that hydro is likely to provide a stronger dose of THC, one might ask whether it is indeed more harmful. Paracelsus would say so: "the dose makes the poison," wrote the 15th century Swiss physician and father of modern toxicology. But some cannabis experts would disagree. They counter that if the cannabis is stronger, then smokers simply smoke less to get high. As Chris Conrad, a court-qualified cannabis expert in El Cerrito, California, put it, "the issue is more like European coffee versus American. In Europe, the coffee is twice as strong, so Americans often ask for it to be watered down, so weak coffee is known there as 'Americano'. But it's still coffee. The same is true for hydro versus outdoor cannabis: it might be stronger, but it's still the same drug." And as the EMCDDA report noted, smoking less to get high may be a benefit since the user takes in less toxic smoke.

Whether hydro is more dangerous because it contains contaminants is tougher to sort out. Urban myths point to everything from nitrogen and chlorophyll (the pigment that makes plants green) to horse manure and pesticides. But chlorophyll and nitrogen are not psychoactive substances, while horse manure and pesticides are more likely to be found in field-grown varieties than in indoor crops.

Still, many anecdotal reports say hydro produces a different effect to field-grown cannabis. One explanation is that the particular growing conditions produce a different spectrum of active cannabinoids – the active substances in cannabis. And that spectrum can effect the quality of the 'high'. It's known, for instance, that an intense high results when the ratio of THC versus other non-psychoactive cannabinoids is large. If it's the reverse, as in the medical extract Sativex, the high is diminished.

But for now, all this is just speculation. Says Hall, "The short, honest answer is that no one knows."

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