1. You buy contact lenses online
One in three people who get their contacts on the Internet don't get eye exams on a regular basis, according to a recent study from Brooklyn College. "As a result, your prescription may be out-of-date, which can lead to eyestrain and chronic headaches," says Thomas Steinemann, M.D., an associate professor of ophthalmology at Case Western Reserve University. Even if you haven't noticed a change in your vision, a yearly checkup is still crucial. "The eye naturally changes shape over time, affecting the fit of the lens," he says. "A tootight pair may irritate or scratch the cornea, leaving you vulnerable to a potentially harmful infection."

2. You pop sleeping pills to help you doze off
For most women, taking an over-the-counter or prescription medicine occasionally is fine. "But you can become dependent on them and require larger doses to help you get to sleep," says Norm Tomaka, a pharmacist in Melbourne, Florida. Sleeping pills work by promoting drowsiness in the brain. Over time, your body relies on that chemical interference, making it harder to fall asleep when you stop taking the sleep aids. Instead of reaching for a pill, try exercising in the morning and switching off the tele vision and computer at least an hour before going to bed to bring sleep on naturally.

3. You sometimes "borrow" your friend's antibiotics
Your throat is killing you and you're sure it's strep again, but you have a big meeting today and can't possibly swing a visit to your doctor's office. So when a coworker graciously offers you a few of her antibiotics, you gratefully accept. (After all, you probably got your illness from her!) Scenario sound familiar? Nearly one in three Americans admits to frequently sharing an Rx with someone else, with antibiotics as one of the most commonly loaned drugs, finds a new study in the American Journal of Public Health.

Experts agree that using a prescription that's not yours is never a good idea. "Antibiotics aren't one-size-fits-all—the right one depends on the kind of infection you have. And your infection may be viral, not bacterial, which renders antibiotics useless," says Tomaka. "They'll only kill off the good bacteria needed to keep your gut healthy." Plus, any time you use someone else's medication you run the risk that it may interact negatively with other drugs or vitamins you're taking.

The fact that anyone even has leftover antibiotics is another health mistake. "When you have an infection, you need an entire course to kill off all the bacteria," says Tomaka. One or two pills may wipe out enough to clear up your symptoms, but any survivors will multiply and possibly lead to a worse infection.