Senna extract is an FDA-approved over-the-counter treatment for occasional constipation. Because there is no controversy regarding senna's effectiveness for this purpose, we do not present the supporting evidence here. Rather, we address the concerns that have been raised regarding senna’s safety.

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, common buckthorn, and turkey rhubarb. The mechanism of action of anthranoids, however, is somewhat worrisome: they seem to work primarily by damaging the cells lining the colon.¹ 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.

Evaluating this possibility is more difficult than it sounds. The most obvious method is to survey a large population over time, and see whether people who use senna have a higher incidence of colon cancer. However, studies of this type ( observational studies) are inherently unreliable, because they don’t show cause and effect. People with colon cancer or other precancerous conditions may become constipated and take senna, and this would cause a statistical association between use of senna and colon cancer, even if senna did not cause cancer. In any case, the results of such studies have been mixed, and overall the association, if any, does not appear to be strong.^2-5

Studies in animals have generally been reassuring, but a few such trials, as well as test-tube studies, have found some evidence of possible increased risk with long-term use.^6-8

Senna does have one potential safety advantage over other herbal anthranoid laxatives: its particular anthranoids are not very absorbable.⁹ This reduces the potential risk of harm deeper in the body.

The bottom line: At present, it appears reasonable to conclude that short-term use of senna is quite safe, while long-term use might or might not be safe. However, senna is not recommended for long-term use anyway. Chronic senna consumption can cause dependency, meaning that it becomes impossible to have a bowel movement without it. In addition, there have been sporadic reports of unusual reactions to chronic use of senna, such as hepatitis.¹⁰

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 additionally 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.

The safety of senna during pregnancy has not been established, and pregnant women are advised to avoid senna during the first trimester.¹¹ Nursing women should also avoid using senna.¹²