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 the 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, caffeine causes the interactions. But other compounds in coffee may also play a role. Studies show that coffee consumption can affect more than a dozen medications as varied as antidepressants, estrogen, thyroid and osteoporosis drugs.

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 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 the 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, that women taking birth control pills held caffeine in the systems four hours longer than women who were not on the pill.