That depends on whom you ask.

Last year, for instance, the Harvard Heart Letter, a publication of Harvard Medical School, said neither flushing old drugs down the toilet nor putting them in the trash was a good method, because people and animals can get into the trash and drugs flushed away might end up in the water supply.

But this year, the White House Office of National Drug Control Policy contradicted much of that with new guidelines telling people to throw old medicines in the trash. It suggested mixing leftover drugs with undesirable substances like coffee grounds or kitty litter (it did not specify whether the kitty litter should be used or unused) and putting the result into containers so drugs could not be “diverted” to unsavory use. The federal guidelines also said some drugs SHOULD be flushed down the toilet, among them painkillers like Actiq and Duragesic Transdermal patches.

But that’s anathema to environmentalists. “We recommend not flushing any [medications] down the toilet,” said Athena Bradley, a projects manager at the Northeast Recycling Council, a nonprofit group with 10 member states. Instead, the group advocates community “take-back” centers or events that allow people to bring unused drugs to collection centers, an approach the government also likes.

Whatever you do, don’t try to return controlled substances like the sleeping pill Ambien to a pharmacy, said Karen Ryle, supervisor of outpatient pharmacy at Massachusetts General Hospital. They are not allowed to accept them because of rules on how drugs should be destroyed and accounted for.