For many Americans, the thought of a morning without coffee is unbearable. More than half of adults drink it regularly, typically about three cups a day.

Most people rarely consider side effects beyond restlessness or trouble falling asleep at night. But coffee and espresso can have other consequences in people taking certain drugs, by either blocking absorption or enhancing their effects.

In many cases, the interactions are caused by caffeine. But other compounds in coffee may also play a role. Studies show that more than a dozen medications — as varied as antidepressants, estrogen and thyroid andosteoporosis drugs — can be affected by coffee consumption.

A study in 2008, for example, found that people who drank coffee shortly before or after taking levothyroxine, a common thyroid medication, experienced a reduction of up to 55 percent in absorption of the drug. Other studies have found that coffee can reduce absorption of the osteoporosis drug alendronate by up to 60 percent, and that it can lower circulating levels of estrogen and other hormones in women.

Some prescription drugs can enhance the effects of coffee and other caffeinated drinks. A number of these drugs, including some antidepressants,antibiotics and birth control pills, block an enzyme known as CYP1A2, which helps metabolize caffeine. As a result, caffeine may persist in the body for several hours longer than normal. One study showed, for example, thatwomen taking birth control pills held caffeine in their systems four hours longer than women who were not on the pill.