St. John's wort interacts with a range of medications. In most cases, this interactions leads to reduced the effectiveness of the medication in question; in other cases, however, St. John's wort may increase the effects of a medication.

If you are currently being treated with any of the following medications, you should not use St. John's wort without first talking to your healthcare provider:

Antidepressants
St. John's wort may interact with antidepressant medications that are used to treat depression or other mood disorders, including tricyclics, SSRIs (see earlier discussion), and monoamine oxidase inhibitors (MAOIs) such as phenelzine. How St. John's wort works is not entirely clear, but is believed to be similar to how SSRIs work. Therefore, using St. John's wort with this class of antidepressants in particular can lead to exacerbation of side effects including headache, dizziness, nausea, agitation, anxiety, lethargy, and lack of coherence.

Digoxin
St. John's wort should not be taken by those on digoxin because the herb may decrease levels of the medication and reduce its effectiveness.

Immunosuppressive medications
St. John's wort should not be taken by those on immunosuppressive medications such as cyclosporine because it may reduce the effectiveness of these medications. In fact, there have been many reports of cyclosporin blood levels dropping in those with a heart or kidney transplant, even leading to rejection of the transplanted organ.

Indinavir and other protease inhibitors
The Food and Drug Administration (FDA) issued a public health advisory in February 2000 concerning the probable interaction between indinavir and St. John's wort that resulted in significantly decreased blood levels of this protease inhibitor, a class of medications used to treat HIV or AIDS. The FDA recommends that St. John's wort not be used with any type of antiretroviral medication used to treat HIV or AIDS.

Loperamide
There has been a report of a possible interaction between St. John's wort and the antidiarrheal medication, loperamide leading to delirium in an otherwise healthy woman.

Oral contraceptives
There have been reports of breakthough bleeding in women on birth control pills who were also taking St. John's wort.

Reserpine
Based on animal studies, St. John's wort may interfere with the intended action of this medication used to treat high blood pressure.

Theophylline
St. John's wort can reduce levels of this medication in the blood leading. Theophylline is used to open the airways in those suffering from asthma, emphysema, or chronic bronchitis.

Warfarin
St. John's wort interferes with the anticoagulant medication, warfarin, by reducing blood levels as well as the effectiveness. This leads to the need to for adjustments in doses of this medication.