Most Americans have a bottle or two of expired medication sitting around. You may be tempted to take this medicine, but concerned about the safety. The Harvard Medical School Family Health Guide reported that 90 percent of drugs tested were found to be safe 15 years after their expiration dates 1. The obvious problem is that you don't want to take the 10 percent of medicines that are no longer safe or effective. There are several reasons to avoid expired prescription drugs.

Loss of Potency

Some drugs lose potency over time and become less effective at treating the intended condition. This is especially true for insulin and nitroglycerin. If your physician thinks you are taking fresh medication but your test results indicate your condition is deteriorating, she might increase the dosage—with harmful results. The U.S. Department of Health and Human Services recommends that you not take expired medication as it may no longer work properly.

Changes in Chemical Composition

Drugs are chemical compounds capable of changing color, odor and texture over time. They can also break down chemically, causing an unintended effect on your body. Though this is a rare occurrence, it is not a chance you want to take especially if the drug expired many years ago.

No Longer Appropriate

Saving old, expired medications is a bad practice for many reasons. Leftover antibiotics taken the next time you get sick can actually worsen your condition if you are infected with a different bacteria. You could also cause the bacteria to become stronger and drug-resistant if you don't take a full course of the antibiotic. Old medications may be contraindicated with drugs you are currently prescribed and neither your physician or pharmacist will know you are taking the expired drugs. Your medical condition may change over time, making certain medicines inappropriate for you. Finally, if you decide to take an expired medication, you may be duplicating the effect of another medicine you are currently taking. It is easy, for example, to take two different medicines that lower your blood pressure without realizing it.