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Exercise may cause you to fail drug test

Rebound effect If you smoke cannabis and then exercise the next day, be warned - you could fail a random drug test.

That's the finding from a team of Australian researchers looking at the long-lasting effect of the active component in cannabis, known as tetrahydrocannabinol (THC).

THC is a lipophilic compound, meaning it dissolves in fat molecules. Previous studies have shown that it can be stored in the body for at least 28 days, with some former heavy cannabis users still showing traces of THC in their body six months later.

Professor Iain McGregor, from the University of Sydney's School of Psychology, says he was prompted into studying the long-lasting effects of THC after hearing numerous cases of people claiming not to have recently used cannabis, yet had high levels of the active compound in their urine or blood.

"The common dominator in all these scenarios was that the people were burning fat," he says.

McGregor, along with researchers from University of Sydney and University of New South Wales wondered whether exercise, which sometimes uses body fat as a source of energy, could release the stored THC back into the blood stream.

Three years ago, they tested their hypothesis on rats. After being given THC, the researchers watched the level of compound in their blood dropped. They were then made the rats exercise for 30 minutes. Blood tests taken immediately after exercise showed that the level of THC increased.

In this latest study, McGregor and colleagues recruited 14 volunteers who were daily cannabis users, typically smoking 3-4 grams of cannabis per week.

Each person rode an exercise bike strenuously for 35 minutes, having refrained from using cannabis since the night before. They had samples taken before, immediately after and two hours after their exercise, to measure the levels of THC in their blood.

They found that the level of THC increased in all of the volunteers after exercise, and in some people it was high enough to register a positive result if they undertook a drug test.

"Someone subjected to workplace or roadside drug testing after a visit to the gym could end up testing positive despite an absence of recent cannabis use," says McGregor.

The researchers also found people with a higher body mass index (BMI), had the highest levels of THC in their blood, regardless of the amount of cannabis they consumed the previous day.

"The more fat you have in your body the bigger the reservoir you have for THC," says McGregor.

Apart from exercise, the researchers note that dieting and stress can also cause the body to use fat reserves, which in turn may increase THC levels in the bloodstream. However, they found that 12 hours of fasting did not increase THC levels.

McGregor says the study could have implications for drug testing in the future.

"If you give a positive result for a drug test, it doesn't necessarily mean you have smoked cannabis recently," he says.

"Someone caught in a bad car accident ... the stress could cause them to burn fat and raise their THC, and they could give a positive drug test."

The researchers plan to repeat the study with more participants and to look at the effect of THC levels after exercise on driving ability and memory.