Senna extract is an FDA-approved over-the-counter treatment for occasional constipation.  It acts as a laxative, increasing the peristaltic (rhythmic, wavelike) movements of the colon.

Senna contains chemicals in the anthranoid family, such as anthraquinones, anthrones, and dianthrones. Related substances are found in a variety of plants used for laxative purposes, such as cascara sagrada and turkey rhubarb.

The mechanism of action of anthranoids is somewhat worrisome. Anthranoids seem to work primarily by damaging the cells lining the colon. In general, cell damage can be a precursor to cancer, and on this basis concerns have been raised that senna might increase colon cancer risk.

Senna does have one potential safety advantage over other herbal anthranoid laxatives. Senna's particular anthranoids are not very absorbable. This reduces the potential risk of harm deeper in the body.

Caution:

Senna should not be used for more than seven consecutive days.  Side effects may include strong cramping and pains in the abdomen.  Senna can cause electrolyte imbalance and loss of body fluids, nausea and rash.  Excessive amounts have been linked to liver toxicity.  Senna should not be taken by children and pregnant or nursing women, as well as people with various gastrointestinal, heart, liver or kidney diseases.  Any absolute safety of senna use during pregnancy has not been established, and pregnant women are advised to avoid senna during the first trimester. Nursing women should also avoid using senna. It is especially important not to overuse senna.

Some experts believe that short-term use of senna is safe, while long-term use might or might not be safe. Senna is not recommended for long-term use. Chronic senna consumption can cause dependency, meaning that it becomes impossible to have a bowel movement without it. If used appropriately, senna should be safe.

As is the case with all laxatives, people with significant colonic disease, such as ulcerative colitis, should not use senna.

If senna is taken to the point of diarrhea, the body may become depleted of the mineral potassium. This is particularly dangerous for people using drugs in the digoxin family, which can cause dangerous cardiac arrythmias if potassium levels in the blood are inadequate. People who use medications that themselves deplete the body of potassium, such as thiazide or loop diuretics, are at special risk of this complication of senna overuse.