[cnn-photo-caption image= http://i2.cdn.turner.com/cnn/2009/images/05/14/art_marijuanaelsohly_cnn.jpg caption= "Mahmoud ElSohly says marijuana's potency will continue to rise before tailing off in the next five years."]

By Jeanne Meserve and Mike M. Ahlers

OXFORD, Mississippi (CNN) - The average potency of marijuana, which has risen steadily for three decades, has exceeded 10 percent for the first time, the U.S. government will report on Thursday.

Scientists working for the government predict that potency, as measured by the drug's concentration of the psychoactive ingredient THC, will continue to rise.

At the University of Mississippi's Potency Monitoring Project, where thousands of samples of seized marijuana are tested every year, project director Mahmoud ElSohly said some samples have THC levels exceeding 30 percent.

Average THC concentrations will continue to climb before leveling off at 15 percent or 16 percent in five to 10 years, ElSohly predicted.

The stronger marijuana is of particular concern because high concentrations of THC have the opposite effect of low concentrations, officials say.

In addition, while experienced marijuana users may limit their intake of potent marijuana, young and inexperienced users may not moderate their intake and possibly suffer from dysphoria, paranoia, irritability and other negative effects.

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