It's part of the U.S. Department of Justice - Drug Enforcement Administration's National Take Back Initiative.
"The National Prescription Drug Take-Back Day aims to provide a safe, convenient, and responsible means of disposing of prescription drugs, while also educating the general public about the potential for abuse of medications," according to its website.
The Citizen's Campaign for the Environment is promoting the campaign.
"Flushing and landfilling medicines are never the right disposal solutions," it said in an email. "Pharmaceutical drugs that are flushed can contaminate local water supplies. Drugs that are thrown out and sent to landfills can contaminate ground and surface waters through leachate.
"National studies have found that pharmaceutical drugs have been detected in drinking water supplies of 41 million Americans. This emerging issue is a threat to public health, wildlife, and our natural environment," according to the CCE. "Properly disposing of pharmaceuticals at drug disposal drop-offs is always a better option than landfilling them or flushing them down the toilet."