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Senna, properly called Cassia senna, is a shrubby herb that is used as a laxative. Senna leaves are brewed into a tea, but its effects are unpredictable because the dose is hard to control.  Senna makes the muscles of the colon contract, which may be accompanied by severe cramping.  If use is prolonged, senna can lead to electrolyte imbalance and laxative-dependence.  Senna must not be used during pregnancy because it is mutagenic and may stimulate endometrial contractions.  In addition to its laxative action, senna is also a mild stimulant and appetite suppressant.  Read more about senna from the Guide to Alternative Medicine at About.com.