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Secondhand marijuana smoke can cause positive drug test

Sit around marijuana smokers in an unventilated room and you may start to feel their high.

A new study by researchers at the Johns Hopkins School of Medicine found that secondhand exposure to cannabis in an unventilated room or enclosed vehicle can cause non-smokers to test positive for the drug in urine tests in some cases.

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It can also cause minor problems with memory and coordination.

The researchers exposed people to secondhand smoke for an hour in an experimental chamber at Johns Hopkins. They then turned off ventilation fans and the non-smokers showed detectable levels of the drug in their blood and urine.

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The results were reported online this month in the journal Drug and Alcohol Dependence.

Similar findings were reported in the 1980s. Researchers then found the drug's active ingredient, tetrahydrocannabinol, or THC, and other cannabis byproducts in the bodies of nonsmokers after an hour or more spent with heavy smokers in an enclosed space.

The Hopkins researchers said they were seeking to update the findings because the average potency of street cannabis has tripled since then. Earlier studies also did not look at whether the nonsmokers reported feeling the drug's effects, the researchers said.

"This study is a significant update in our knowledge of cannabis smoke effect on nonsmokers and has implications in many arenas, including drugs and driving," co-author Edward J. Cone,  a Johns Hopkins adjunct professor of psychiatry and behavioral sciences who performed the studies in the 1980s, said in a statement.

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Researchers pointed out the study was limited because the sample was small and it did not have a placebo trial using cannabis that contained no THC.

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