Her mood is better. Her birth control may not be working.

The herb is working, which is exactly why she wants to keep taking it. The spotting is the clue that it may be working on her contraception too.

The mechanism

St. John's wort induces CYP3A enzymes, the same enzymes that clear ethinyl estradiol and norethindrone. Faster clearance means lower hormone exposure across the cycle.

What the studies found

In a study of 12 women on a combined pill, adding St. John's wort at 300 mg three times daily cut the half-life of ethinyl estradiol roughly in half and increased clearance of norethindrone. Breakthrough bleeding occurred in 7 of 12 women during the herb phase, compared with 2 of 12 without it.

A second study of 16 women on a low-dose pill found a 13 to 15 percent drop in hormone exposure. It also found more breakthrough bleeding, follicle growth, and evidence of probable ovulation.

Breakthrough bleeding is the symptom. Reduced contraceptive protection is the risk.

What the evidence says

Both studies are small and measured pharmacokinetics and bleeding, not pregnancies. Still, the mechanism is well established, the direction is consistent, and the consequence of being wrong is an unplanned pregnancy. Most guidance treats this as a real interaction.

What to do Monday

What to do

1. Ask about over-the-counter herbs at every visit. Patients rarely volunteer them.
2. If she continues St. John's wort, recommend backup contraception or a method less affected by enzyme inducers, such as an IUD.
3. Revisit the mood plan. If she needs an antidepressant, stop the herb first.