WASHINGTON —  Marijuana is getting a little more potent each year. A project at the University of Mississippi has tested samples of marijuana seized by law enforcement agents annually since the late 1970s to check levels of THC, the active ingredient.

The testing has long been the basis for government warnings that marijuana is potentially more harmful today than it was for previous generations, an assertion disputed by critics of marijuana laws.

The average amount of THC in marijuana seized last year reached 8.5 percent, continuing an almost yearly uptick, the White House drug-control policy office said Wednesday. The THC level has doubled since the 1980s.

The government estimates that 4.1 million Americans use marijuana. Use by teenagers has declined recently, but federal officials worry that marijuana is being cited more often in emergency room visits.