Note: The following is a cursory review of milk thistle and is presented for informational purposes only. It is not intended as medical advice. Please discuss taking this or any supplement with your doctor prior to taking it.
Milk thistle is generally considered safe and typically has few side effects; however, if the side effects of milk thistle make Gleevec less tolerated (there may be some concern about diarrhea and nausea), then it would argue against taking milk thistle.
According to the NCI (National Cancer Intitute), the evidence for (and against) milk thistle is inconclusive, but “Human studies of silymarin (the active ingredient in milk thistle) have shown minimal adverse effects in multiple large blinded, placebo-controlled, randomized studies.” Also according to the NCI, “It is not known if milk thistle may make anticancer medications or other drugs more effective, less effective, or have no effect when taken with them.”
There seems to be some concern that milk thistle can inhibit a liver enzyme called CYP3A4. This is the enzyme that metabolizes Gleevec and Sutent. In theory, this could cause an increase in Gleevec or Sutent blood levels (more toxicity). We looked at several different studies that examined the effect of milk thistle on the metabolism of other drugs metabolized by CYP3A4 and the results were inconclusive; several found little or no effect.
Overall, milk thistle seems pretty safe and there is some evidence of beneficial effects (but overall it’s inconclusive). Before considering milk thistle, check with your doctor and monitor for any signs of increased toxicity, especially nausea, diarrhea and any unacceptable increase in Gleevec-related side effects.
The overall opinion of the NCI was “Given the limited amount of human data, the use of milk thistle/silymarin as a treatment for cancer patients cannot be recommended outside the context of well-designed clinical trials.”