Q Over the years, we have had many cats and dogs who each, at one or more times in their lives, had surgery. Mostly neutering or declawing, but a few had more serious surgeries and were treated for other illnesses.

Each time, pain medications such as morphine were prescribed, but we never seem to use them all up. As a result, we have a considerable stock of unused and expired medications. Can these be donated to a shelter or vet clinic for stray animals or financially strained owners?

A: As you are well-aware, the only person who is more concerned about the comfort of a post-surgical patient than the veterinarian is the owner. As a result, we tend to err on the side of over-treating pain, so frequently there are leftover medications. Unfortunately, these are often controlled substances and cannot legally be donated and administered to another patient. Once a drug of any type leaves a hospital, it cannot be reapplied to another patient, as its quality cannot be assured and there are professional and legal rules prohibiting this.

So what do we do with leftover medications? Flushing them down the toilet has been a traditional approach, but in an increasingly crowded world, we have come to realize that today's wastewater is tomorrow's drinking water.

The use of prescription and nonprescription drugs continues to grow worldwide. An IMS Institute for Healthcare Informatics study totaled 3.99 billion retail prescriptions in 2010 in the U.S.

Veterinary drugs are also playing a role. Steroids administered to cattle have been shown to pass through the animals unchanged and show up in nearby streams. Companion animal drugs are also prescribed in large amounts. Animal Health Institute statistics indicate that in 2008 $5.2 billion worth of veterinary medications were dispensed.

Human drugs and their metabolites end up in the water supply primarily through the toilet, either as they pass through the person or are discarded. Drugs given to animals also can enter the water supply through runoff water as they are passed through the animal's system.

Though the long-term effects of low-level exposure to these drugs on human health and wildlife is uncertain, the fact that the exposure is widespread is not.

An Associated Press study indicated that more than 41 million Americans are exposed to hormones, antibiotics, antipsychotics and seizure medications in their drinking water. This is a worldwide phenomenon, as contamination also has been documented in Asia, Europe and Australia.

Although the amounts measured are in concentrations far below their therapeutic levels, sometimes in parts per trillion, we really don't know how this will affect us or other organisms over years of exposure. In a 2008 USA Today article, Mary Buzby, director of environmental technology at the drug company Merck & Co., said that even at small concentrations the drugs “could be causing impacts to human health or aquatic organisms.”

The article goes on to describe laboratory studies in which low-level exposure to certain medications has enhanced the growth of breast cancer cells, slowed the growth of embryonic kidney cells and caused blood cells to demonstrate signs of inflammation. Studies also have demonstrated effects on wildlife such as feminization of male fish and negative effects on zooplankton and earthworms.

We have just installed a device called the Smart Sink, which renders liquid and pill drugs unuseable and unrecoverable. The device is tamper-proof, sealed and may be disposed of in the trash or incinerated. Clients are welcome to drop off their unused drugs for disposal.

Recommendations for at-home disposal are to mix drugs with things such as coffee grounds or kitty litter, preferably used. Place the mixture in a sealed container and then in the trash. Alternatively, many pharmacies will take unused drugs for disposal.

Dr. Henri Bianucci and Dr. Perry Jameson are with Veterinary Specialty Care LLC. Send questions to petdocs@postandcourier.com.