The respected Trimbos Institute said that several of the pills have been handed in recently at a network of centers that test drugs for users before they take them.
Many of the Americans still missing since Wednesday, officials and relatives said, were on a trek to Mount Kailash, which is in Tibet and revered by Hindus and Buddhists.
At least 469 people have been killed and more than 1,000 are missing in catastrophic floods that swept through the Himalayan region. More than 500 foreigners, including 90 Americans, are among those missing.