St. John's wort interacts with a large number of medications. In most cases, St. John's wort decreases the effectiveness of the medication; 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 doctor:

Antidepressants -- St. John's wort may interact with medications used to treat depression or other mood disorders, including tricyclic antidepressants, SSRIs, and monoamine oxidase inhibitors (MAOIs). Taking St. John's wort with these medications tends to increase side effects, and could potentially lead to a dangerous condition called serotonin syndrome. Do not take St. John's wort with other antidepressants, including:

• SSRIs: Citalopram (Celexa), escitalopram (Lexapro), fluvoxamine (Luvox), paroxetine (Paxil), fluoxetine (Prozac), sertraline (Zoloft)
• Tricyclics: Amitriptyline (Elavil), nortriptyline (Pamelor), imipramine (Tofranil)
• MAOIs: Phenelzine, (Nardil), tranylcypromine (Parnate)
• Nefazodone (Serzone)

Allergy drugs (antihistamines) -- St. John's wort may reduce levels of these drugs in the body, making them less effective:

• Loratadine (Claritin)
• Cetirizine (Zyrtec)
• Fexofenadine (Allegra)

Dextromethorphan (cough medicine) -- When taken at the same time as dextromethorphan, a cough suppressant found in many over-the-counter cough and cold medicines, St. John's wort can increase the risk of side effects, including serotonin syndrome.

Digoxin -- St. John's wort may decrease levels of the medication and reduce its effectiveness. Do not take St. John's wort if you take digoxin.

Drugs that suppress the immune system -- St. John's wort can reduce the effectiveness of these medications, taken after organ transplant or to control autoimmune diseases. 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.

• Adalimumab (Humira)
• Azathioprine (Imuran)
• Cyclosporine
• Etanercept (Enbrel)
• Methotrexate
• Mycophenolate mofetil (CellCept)
• Tacrolimus (Prograf)

Drugs to fight HIV -- St. John's wort appears to interact with at least two kinds of medications used to treat HIV and AIDS: protease inhibitors and non-nucleoside reverse transcriptase inhibitors. The Food and Drug Administration recommends that St. John's wort not be used with any type of antiretroviral medication used to treat HIV or AIDS.

Birth control pills -- There have been reports of breakthrough bleeding in women on birth control pills who were also taking St. John's wort, and it is possible that the herb might interfere with the effectiveness of birth control pills, leading to unplanned pregnancies.

Reserpine -- Based on animal studies, St. John's wort may interfere with reserpine's ability to treat high blood pressure.

Sedatives -- St. John's wort can increase the effect of drugs that have a sedating effect, including:

• Anticonvulsants such as phenytoin (Dilantin) and valproic acid (Depakote)
• Barbiturates
• Benzodiazepines such as alprazolam (Xanax) and diazepam (Valium)
• Drugs to treat insomnia, such as zolpidem (Ambien), zaleplon (Sonata), eszopiclone (Lunesta), and ramelteon (Rozerem)
• Tricyclic antidepressants such as amitriptyline (Elavil)
• Alcohol

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

Triptans (used to treat migraines) -- St. John's wort can increase the risk of side effects, including serotonin syndrome, when taken with these medications:

• Naratriptan (Amerge)
• Rizatriptan (Maxalt)
• Sumatriptan (Imitrex)
• Zolmitriptan (Zomig)

Warfarin -- St. John's wort reduces the effectiveness of warfarin, an anticoagulant (blood-thinner).

Other drugs -- Because St. John's wort is broken down by certain liver enzymes, it may interact with other drugs that are broken down by the same enzymes. Those drugs may include:

• Antifungal drugs, such as ketoconazole (Nizoral), itraconazole (Sporanox), fluconazole (Diflucan)
• Statins (drugs taken to lower cholesterol)
• Some calcium channel blockers (taken to lower blood pressure)