St. John's wort and cancer medicines: a serious interaction

This page is for people having cancer treatment who take, or are thinking about taking, St. John's wort. It explains why this herb matters more than most supplements.

Most articles about supplements and cancer end with a shrug: the evidence is thin, talk to your doctor. This is not one of those.

NCCIH (the National Center for Complementary and Integrative Health, part of NIH) uses unusually strong language about St. John's wort. It says the herb can interact in dangerous, sometimes life-threatening ways with a variety of medicines. That is a plain warning, not a hedge.

If you are on cancer treatment and taking St. John's wort, this belongs in a phone call today rather than at your next scheduled visit.

NCCIH names specific cancer medicines whose effects St. John's wort weakens: irinotecan, imatinib and docetaxel.

Think about what "weakens" means in this setting. A cancer drug is prescribed at a dose calculated to do a particular job. If a cancer drug is weakened, the treatment may not work as well as planned, and you may not notice.

This is a very different kind of harm from a side effect. A side effect announces itself. A weakened treatment does not.

Cancer drugs are not the only medicines affected. NCCIH lists interactions with:

• Birth control pills
• Heart medicines
• HIV drugs
• Anticoagulants, which are blood thinners
• Seizure medicines
• Certain antidepressants

There is a second kind of risk with antidepressants specifically. Combining St. John's wort with drugs that affect serotonin may lead to increased serotonin-related side effects, which NCCIH says can be serious.

That combination is easy to stumble into, because someone taking a supplement for low mood may also be prescribed something for low mood. Two things aimed at the same target can add up in a way neither was designed for.

St. John's wort is a plant with yellow flowers, used for conditions including depression, stomach ulcers and colds, and to aid wound healing. Its name refers to John the Baptist, because it blooms around his feast day in late June.

On effectiveness, NCCIH reports that research indicates it may be helpful for mild or moderate depression. For those conditions, it appears comparable to standard antidepressants. Evidence for severe depression, and for use beyond 12 weeks, remains uncertain.

Small numbers of studies suggest possible benefit for menopausal hot flashes and somatic symptom disorder (a condition involving intense anxiety about physical symptoms). It has also been studied for ADHD, irritable bowel syndrome, obsessive-compulsive disorder and smoking cessation.

So this is not a useless product. It is an active one, and being active is exactly why it interferes with other active substances.

For adults not taking medications, NCCIH says it appears safe for up to 12 weeks. The side effects it lists include diarrhea, dizziness and sleep disturbances, and sun exposure after use may cause severe skin reactions. NCCIH advises pregnant people to avoid it because of the risk of birth defects.

If you are having radiation treatment, mention any skin reactions to your team.

If this applies to you, the steps are short.

• Tell your oncology team, and say when you started and how much you take.
• Bring the actual bottle to the appointment so they can see the product.
• Do not stop or change anything else on your own; let them advise on sequencing.
• Mention it to your pharmacist too, who can check your full medicine list.

And if low mood is what led you to the supplement in the first place, say that as well. Depression during cancer treatment is common and treatable, and your team can offer options that will not undercut the treatment itself.