While chalk itself is not toxic, there is a potential danger in eating anything that is not actually food due to the fact that it is not processed according to food industry regulations and may contain any number of impurities. About.com warns that it can also be dangerous if done excessively because malnutrition is likely to result.
Chalk is mostly calcium, which is not dangerous. In fact, it is a necessary nutrient found in dairy products and dark leafy green vegetables. However, the desire to eat chalk itself could be a sign of an eating disorder called pica, according to HowStuffWorks.com. People who suffer from pica have the persistent desire to eat substances that are not food. Among the most common examples are dirt, chalk, clay and ash, but some pica sufferers may crave hair, soap, paint and even glass. Pica is more common among pregnant women; there is some thought that the craving for minerals may be the body's response to the mineral deficiencies that can accompany pregnancy. However, one of the most common cravings of pregnant women is ice, which contains nothing nutritive except water. While some of the substances pica patients consume are relatively harmless, the compulsion itself could be cause for concern and sufficient reason to seek a physician's advice.