Jeremy Piven suffered "shocking levels" of mercury in his system from eating too much sushi and Chinese herbs, forcing him to leave the Broadway play Speed-the-Plow, his doctor tells PEOPLE.

After a battery of tests failed to reveal what was ailing the three-time Emmy winner, Colker checked his "heavy metals" and was "absolutely stunned" to find mercury at a level "almost six times the upper limit of normal and allowable," says the doctor. "It's the highest level I've ever seen."

Colker attributes the high mercury count to Piven's habit of eating sushi, often twice a day, compounded by certain Chinese herbs he was taking "for general health." Piven was ordered to put a moratorium on the fish and the herbs, and his doctor sent a letter on Dec. 10 revealing the health problem to the Speed-the-Plow production staff.