The average American adult drinks about three cups of coffee a day, and while many can't imagine a morning without it, people taking certain drugs may want to think twice.

Studies show that more than a dozen medications — antidepressants, estrogen, and thyroid and osteoporosis drugs, for example — can be affected by coffee, which can block absorption or enhance the effects of the drugs.

One study found that people who drank coffee shortly before or after taking levothyroxine, a common thyroid medication, absorbed 55 percent less of the drug. The osteoporosis drug alendronate was similarly affected by coffee consumption.

Other prescription drugs can actually enhance the effects of caffeine by blocking an enzyme that helps metabolize it, making caffeine stay in the body several hours longer than it otherwise would. Case in point: research shows women taking birth control pills can hold caffeine in their systems up to four hours longer than other women.

The bottom line? You may want to ask your doctor if your morning cup of java will interact with any of your prescribed medications.

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