There are a number of known and potential milk thistle side effects that should be considered before taking this herb, including some isolated side effects and a few possible negative drug interactions. Some known side effects include nausea, diarrhea, indigestion, and loss of appetite. Milk thistle is a plant within the daisy family and gains its name both from the white splotches that appear on the leaves and the white sap contained within the plant. The leaves are often eaten in salads as a replacement for lettuce or spinach, and the seeds can be ground up and used in beverages in place of coffee beans. There are also milk thistle extracts that can be used as dietary supplements, usually intended to combat or prevent liver damage.
While milk thistle can cause a number of unpleasant intestinal and digestion related issues, it can also be used as a natural laxative. Milk thistle is likely safe for most healthy adults, but if any side effects do occur, the person should stop taking it immediately and consult a medical professional before further use. There is little research regarding possible side effects for women who are pregnant or nursing, and it should probably be avoided during such times.
Milk thistle can also cause an allergic reaction in people who are allergic to plants of the Asteraceae/Compositae family, including ragweed, chrysanthemums, and daisies. Anyone with allergies to these plants should avoid consuming milk thistle without first consulting a medical professional.
Other milk thistle side effects can come from extracts of the plant acting like estrogen in some people. Anyone with a condition that could be made worse from the effects of increased estrogen, such as certain types of cancer, should avoid these types of extracts. Extracts made from milk thistle seeds, however, have not been found to have these effects. The plant has also been shown to reduce the effectiveness of estrogen hormone treatments, since it seems to help the body break down estrogen supplements faster, so taking both can reduce the amount of estrogen actually gained from the treatments.
Known side effects of milk thistle include the potential to interact with medications used for lowering cholesterol. The plant may affect how long the medication stays in a person's body, which can increase or decrease its effectiveness. Anyone taking cholesterol lowering medication should consult a healthcare professional before taking milk thistle.
Though milk thistle is used to help people who have livers that have been damaged through exposure to toxic chemicals, it should not be taken with drugs that are broken down for the body by the liver. Research has shown that milk thistle can slow the breakdown of these drugs, extending how long they are effective in the body. This can lead to an accidental overdose or can increase the period of time in which negative interactions between medications can occur.