Our leftover medicines are spawning drug-resistant "bacterial monsters" that thrive inside sharks, scientists say.

The finding suggests antibiotics such as penicillin may be leaching into the environment and spurring drug-resistant bacteria to evolve and multiply in the oceans.

"Bacteria have sex, basically. They can transfer genetic material," said study leader Mark Mitchell, professor of veterinary clinical medicine at the University of Illinois at Urbana-Champaign.

Mitchell and colleagues found antibiotic-resistant bacteria in seven species of shark—such as bull sharks, lemon sharks, and nurse sharks—as well as the redfish Sciaenops ocellata. The fish live in coastal waters off Belize, Florida, Louisiana, and Massachusetts. (See shark pictures.)

Though random mutations can account for the drug-resistant bacteria, there's ample evidence for human origin, he noted.

"What do people do with antibiotics when they don't finish them? They flush them down the toilet [or] put them in the garbage," Mitchell said.

Trashed Medicines Making Monsters

Bacteria exposed to the drugs develop resistance, Mitchell said, so "we have the risk of creating these bacterial monsters."

These monsters may cause particularly virulent illnesses in sharks and fish. But the researchers are also concerned the resistant bacteria will find their way back into the human food chain.

Though sharks aren't a staple in the human diet, we eat what they eat—crab, shrimp, and other fish. So people should be aware of these risks and handle food appropriately to avoid infection, Mitchell cautioned. (Check out Green Guide's tips for buying healthy fish.)

"I will eat things like sushi," he said. "But knowing there are those types of risks, I'm going to try and get it from healthy, wild-caught fish, where there might be more of a minimal exposure [to drugs]."