The following is part of a research paper by the University of Maryland
Licorice consumption may cause serious side effects. Too much of the component glycyrrhizin causes a condition called pseudoaldosteronism, which can cause an individual to become overly sensitive to a hormone in the adrenal cortex. This condition can lead to headaches, fatigue, high blood pressure, and even heart attacks. It may also cause water retention, which can lead to leg swelling and other problems. An overdose of glycyrrhizin can lead to harmful conditions such as high blood pressure and even heart attack.
Although the most dangerous effects generally only occur with high doses of licorice or glycyrrhizin, side effects may occur even with average amounts of licorice. Some people experience muscle pain or numbness in the arms and legs. Too much licorice can also cause weight gain. These problems can probably be avoided if dosages are kept within the recommended guidelines. It is safest, though, to have use of licorice monitored by your health care provider.
People with high blood pressure, obesity, diabetes, or kidney, heart, or liver conditions should avoid licorice. This herb should also not be used by pregnant or breast-feeding women or by men with decreased libido or other sexual dysfunctions. Use of any licorice product is not recommended for longer than 4 - 6 weeks.