When you’re sick, you’ll generally take whatever prescription your doctor writes up. After all, he’s the one who went to med school. But failing to ask him a few questions could result in a serious dose of unnecessary antibiotics, according to a new study in the Journal of Antimicrobial Chemotherapy.

Researchers examined two major medical surveys over 2 years and found that 60 percent of the time, doctors chose the strongest types of antibiotics when doling out prescriptions. In 25 percent of those cases, the drugs were prescribed for viruses, which can’t be treated with antibiotics.

That’s a problem because strong antibiotics—often referred to as broad spectrum—can kill “good” bacteria, which could trigger future problems like eczema, asthma, inflammatory bowel disease, and obesity, says study coauthor Adam L. Hersh, M.D., Ph.D., of the University of Utah. “When these antibiotics are overused, they’re less likely to work in the future when they’re needed,” Hersh says.(And while you're at it, make sure you know the Top 10 Questions Every Man Must Ask His Doctor)

So how can you know if you really need that one-a-day knockout pill?

1. You don’t always need a doctor.

Viruses don’t respond to antibiotics, and most of the time infections go away on their own, says Men’s Health family medicine advisor Ted Epperly, M.D. If your fever is less than 102 degrees and you can function pretty well, wait and see if the usual symptoms—fatigue, headache, sore throat, diarrhea—get worse before scheduling an appointment.

2. Ask for a narrow spectrum antibiotic—if your condition calls for it.

Docs most commonly over-prescribe antibiotics for upper respiratory conditions like coughs, colds, and bronchitis. If you’ve got something less common, like a skin infection, ask your M.D. for a narrow spectrum antibiotic. (Standard options include penicillin and cephalosporin, or Keflex.) In fact, request the narrowest spectrum possible for the shortest period of time, advises Dr. Epperly. Doctors will prescribe for 10- to 14-day cycles, when in reality you might only need 3 to 5 days, he says.(Give your airways a break by avoiding these 5 Health Threats to Your Lungs.)

3. Be upfront with your doc.

Most physicians feel like they have to do something for you or you won’t be happy, which can lead to an unnecessary antibiotic, says Dr. Epperly. Say, “If you think this might be a virus, it’s okay with me if I don’t get an antibiotic.” That will open up the door for your doc to say, “No, I don’t think you really need one” or “Let’s give it a few days.”

4. But don’t be pushy.

You might think antibiotics have magical properties, but don’t nudge your doctor into giving you a pill if he doesn’t think it’s necessary. Doctors often cite patient pressure as the reason for prescribing inappropriately, Hersh says. Don’t be the guy saying, “Can I get a Z-Pak today?” if your doesn’t doc think it’s a good idea. (Do the research and get the truth about your doctor's next prognosis. Learn the 8 Things Your Doctor's Not Telling You.)