Above this level, users may suffer from chronic insomnia, persistent anxiety and depression, and stomach ulcers. Caffeine use appears to be associated with irregular heartbeat and may raise cholesterol levels, but there is no firm evidence that caffeine causes heart disease. The evidence is also unclear concerning caffeine and cancer. Caffeine and some of its metabolites can cause changes in the cells of the body and in the way in which they reproduce themselves, and caffeine certainly enhances this kind of action by some known carcinogens. However, although caffeine is suspected as a cause of cancer, the evidence is contradictory and does not allow a clear conclusion.
Some animal studies suggest that caffeine can have anti-cancer properties.
Caffeine certainly has the ability to cause a variety of reproductive effects in animals, including congenital abnormalities and reproductive failures, reduced fertility, prematurity, and low birth weight. What is unknown is whether these findings are relevant to the use of ordinary amounts of caffeine-containing beverages by pregnant women. Pregnant women have been advised to restrict caffeine intake by both Canadian and United States governments. Pregnant smokers should be especially wary. The most common medicinal use of caffeine is as a part of headache preparations and other pain relievers.