Your head aches; your nose is stuffy; and your throat is raw. Rummaging around in the medicine cabinet for something to help you feel better, you come across a leftover prescription and wonder: 'How long do antibiotics last anyway?' And in that miserable moment of sickness, you figure it can't hurt and dose yourself with whatever antibiotic you have on hand. A new study shows you aren't alone.

In fact, one out of four people say that they would use antibiotics without their doctor’s okay according to a recent survey of 400 patients treated in primary-care clinics in Houston, Texas. And five percent of respondents admitted to doing exactly that in the past year.

In the study, published July 11 in the journal Antimicrobial Agents and Chemotherapy, 14 percent of respondents also told researchers that they keep a stash of antibiotics at home. People who actually used antibiotics without a prescription reported getting them from a variety of sources—leftovers from a previous illness, friends or family members, or outside the U.S. A few even resorted to taking drugs intended for the family pet.

In most cases, the antibiotics likely didn't help. People most commonly used the drugs to treat cold and sinus symptoms, which typically clear up without medication.

“Respiratory symptoms such as a cough, sore throat, or runny nose are mostly caused by viruses and antibiotics don't work against viruses,” says Larissa Grigoryan, M.D., an instructor in the department of Family and Community Medicine at the Baylor College of Medicine in Houston and one of the study authors. Antibiotics do kill bacteria, but it takes different drugs for different bugs. So if even you have a bacterial infection says Grigoryan, taking leftover antibiotics may not work against the particular bug that’s making you sick.

She also adds a caution about pet meds: “People and animals don't respond to common medications in the same way," says Grigoryan. Antibiotics a veterinarian prescribed for Fluffy or Fido may not be effective against human illness or could even prove harmful.

While taking leftover antibiotics probably won’t help, Grigoryan points out that they can hurt you. Antibiotics commonly cause side effects such as nausea, vomiting, and diarrhea. "More rarely, they trigger allergic reactions and cause more serious side effects," says Grigoryan. For example, they can kill off protective bacteria in your intestines, leaving you vulnerable to infections from bacteria such as clostridium difficile (C. diff), which can be life-threatening. 

What’s more, overuse and misuse of antibiotics has promoted the rise of dangerous superbugs—strains of bacteria that have acquired resistance to multiple antibiotic drugs.

Don't Use Leftover Antibiotics

"It really doesn't matter how long antibiotics last because—expired on not—you should never bypass your doctor to dose yourself with leftover drugs," advises Marvin M. Lipman, M.D., Consumer Reports' chief medical advisor. "That's true even if your symptoms seem similar to those of a previous illness."

If you're sick enough to need more than chicken soup and over-the-counter medications, call your healthcare provider. When an antibiotic is warranted, take all the medication prescribed for you unless your healthcare provider advises you to stop.

And don't hang on to leftovers. Discard unused antibiotics by returning them to the pharmacy or a community take-back program. Or mix the medication with an unpalatable substance such as kitty litter or coffee grounds, seal it in a bag, and throw it out with the household trash. Read more about safe ways to dispose of antibiotics and other unwanted medicines.

Editor's Note: These materials are made possible by a grant from the state Attorney General Consumer and Prescriber Education Grant Program, which is funded by the multi-state settlement of consumer-fraud claims regarding the marketing of the prescription drug Neurontin (gabapentin).