Still, about 3.9 billion prescriptions were filled at pharmacies alone in 2013, according to the Kaiser Family Foundation.

“They only removed an infinitesimal fraction of the reservoir of unused drugs that are out there,” said Dr. Nathaniel Katz, an assistant professor of anesthesia at Tufts University School of Medicine in Boston who studies opioid abuse. “It’s like trying to eliminate malaria in Africa by killing a dozen mosquitoes.”

Dr. Katz is optimistic that the D.E.A.’s decision could have a powerful impact. Putting drop-off receptacles for controlled substances in pharmacies will mean consumers have year-round access to disposal services.

It would be a “very positive” development if such access gets consumers in the habit of returning unused drugs to the pharmacies from which they were obtained, Dr. Katz said.

“It’s more likely to accomplish the objective of minimizing the reservoir of potentially fatal medications in our medicine cabinets than can be accomplished by intermittent programs,” he said.

Flushing controlled substances, especially prescriptions that might kill a child or pet with a single dose, remains an option for consumers, as is throwing out other prescriptions in zipped plastic bag mixed with cat litter, but both are discouraged because of environmental concerns.

The Environmental Protection Agency favors disposal through drug take-back programs over flushing to keep medicines from entering streams and rivers. Yet the Food and Drug Administration recommends flushing unused medications when the potential for harm to someone in the household is great.