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Mischoulon and Rosenbaum detail the gradually developing concern over drug-drug
interactions with St. John’s wort: “These interactions, occurring largely via the liver
enzyme CYP- 450 - 3A4, have resulted in decreased activity of several drugs, including
warfarin, cyclosporin, oral contraceptives, theophylline, fenprocoumon, digoxin,
indinavir, and irinotecan HCl injection (Camptosar).”^18
The Natural Standard states that: “St. John’s wort interferes with the way the body
processes many drugs using the liver’s cytochrome P450 enzyme system. As a result, the
levels of these drugs may be increased in the blood in the short term (causing increased
effects or potentially serious adverse reactions) and/or decreased in the blood in the
long term (which can reduce the intended effects).”^19 Examples of medications that may
be affected in this matter include: carbamazapine, cyclosporine, which prevents the
body from rejecting transplanted organs, irinotecan (Camptosar) and possibly other
drugs used to treat cancer, midazolam, nifedipine, theophylline (dimethylxanthine),
fenprocoumon (Marcoumar, Marcumar), statins, anesthetics (undefined, presumably
general), anticoagulants like warfarin (Coumadin) and related anticoagulants, and
human immunodeficiency virus drugs such as nonnucleoside reverse transcriptase
inhibitors (NNRTIs), protease inhibitors, indinavir (Crixivan) and possibly other drugs
used to control HIV infection.
The US Food and Drug Administration suggests that people with HIV/AIDS taking
protease inhibitors or on NNRTIs avoid taking St. John’s wort.
Extreme caution is urged for cancer patients receiving chemotherapy and for
transplant and other patients who take immunosuppressive drugs.
It must be presumed that, given the popularity of St. John’s wort, many people are
combining it with standard antidepressants without telling their doctors. Clinicians
should advise their patients not to combine St. John’s wort with tricyclic
antidepressants, SSRIs or MAOIs, because anecdotes of "serotonin syndrome" have
been reported, presumably related to St. John’s wort's MAOI activity. See Major Risks,
above. In addition, when combined with certain antidepressants, St. John’s wort may