However, habitual use, even in small doses, can lead to dependence and withdrawal symptoms. These doses can lead to anxiety, irritability, disturbed sleep patterns, tremors, and nervousness. Caffeine use is discouraged for people with certain health conditions. For example, people with gastric ulcer disease are discouraged from consuming caffeinated products because they stimulate gastric secretions that may lead to ulcer formation. In addition, caffeine can interact negatively with certain medications, such as lithium.
Caffeine can also alter the body's metabolism of some nutriments including calcium and iron.
Doctors advise woman to avoid caffeine during pregnancy and lactation. Essentially, caffeine enters the bloodstream and crosses the placenta, reaching the fetus. This is of concern because the fetus has limited ability to metabolize caffeine. It also stays in the bloodstream during pregnancy, particularly during the second and third trimesters. Some animal studies show a significant risk of birth defects caused by caffeine consumption during pregnancy.