The company said on Wednesday that the solution is called “DisposeRx” and consists of a packet with ingredients that, when combined with warm water and emptied into a pill bottle, neutralize leftover medications so they can be disposed of without worry. The packets are free of charge and available at 4,700 Walmart and Sam’s Club stores with pharmacies.

Citing data from the Substance Abuse and Mental Health Services Administration and the National Institute of Drug Abuse, Walmart said some 65% of opioid abusers get them from unwitting family and friends. And about one third of medications sold go unused, making them more readily available for abuse.

As Fortune reported in an in-depth report on the crisis in September, an American is now far likelier to die of a drug overdose than in a car crash. In 2015, opioid overdoses claimed 91 lives a day in the U.S.

While drug distributors like McKesson have borne the brunt of criticism, pharmacy operators have faced pressure as well to step up their role in fighting the crisis. A few months ago, CVS Health, (CVS) the largest dispenser of drug prescriptions in the country ($61 billion worth in 2016) said it would limit the prescription of opioids for some conditions to seven days, a move Walmart has also advocated.

Walmart has a 5% share of the U.S. prescription market, according to Pembroke Consulting, having filled $20.6 billion worth of scripts in 2016, putting it on par with No. 3 U.S. drugstore chain Rite Aid.

DisposeRx packets have a polymer blend that when combined with warm water in the pill bottle sequesters any type of prescription drug, whether a powder, pills, tablets, capsules, liquids or patches, into a biodegradable gel.

Starting Wednesday, patients getting a new Class II opioid prescription filled at a Walmart pharmacy will get a free DisposeRx packet. Those with chronic Class II opioid prescriptions will be able to get one every six months.