St. John's wort (Hypericum perforatum) is a flowering shrub native to Europe. It gets its name from the fact that it often blooms on the birthday of the biblical John the Baptist.

The flowers and leaves of St. John's wort contain active ingredients such as hyperforin. St. John's wort is available as a supplement in teas, tablets, liquids and topical preparations.

People use St. John's wort to treat depression and menopausal symptoms.

Research on St. John's wort use for specific conditions shows:

• Depression. Several studies support the therapeutic benefit of St. John's wort in treating mild to moderate depression. In fact, some research has shown the supplement to be as effective as several prescription antidepressants. It's unclear whether it's beneficial in the treatment of severe depression. Because St. John's wort interacts with many medications, it might not be an appropriate choice, particularly if you take any prescription drugs.
• Menopausal symptoms. Some evidence suggests that taking St. John's wort alone or in combination with black cohosh or other herbs might reduce menopausal symptoms such as hot flashes.
• Somatic symptom disorder. Some studies indicate that St. John's wort might be beneficial for the treatment of this condition that causes severe anxiety about physical symptoms such as pain, weakness or shortness of breath.

When taken orally for up to 12 weeks in appropriate doses, St. John's wort is generally considered safe. However, it may cause:

• Agitation and anxiety
• Dizziness
• Diarrhea, constipation and stomach discomfort
• Dry mouth

Other side effects may include:

• Fatigue and insomnia
• Headache
• Increased sensitivity to sunlight (photosensitivity)

There isn't enough information about the safety of using St. John's wort topically.

Don't use St. John's wort during pregnancy or while breastfeeding.

The list below includes some recognized interactions. However, other drugs may also interact with St. John's wort. Check with your doctor before using St. John's wort in combination with any other drugs, especially prescription medications.

• Alprazolam (Xanax). Taking St. John's wort with this drug used to relieve symptoms of anxiety might decrease the drug's effect.
• Antidepressants. Taking St. John's wort with antidepressants might increase the risk of the accumulation of high levels of serotonin in your body. Too much serotonin can cause mild to severe side effects. Taking this supplement and an antidepressant requires a doctor's supervision.
• Barbiturates. Taking St. John's wort with a drug that acts as a central nervous system depressant (barbiturate) might decrease barbiturate-induced sleep time.
• Bupropion (Wellbutrin SR, Forfivo XL). Taking St. John's wort with this antidepressant might decrease the drug's effect.
• Certain chemotherapy drugs. Taking St. John's wort with irinotecan (Camptosar, Onivyde), docetaxel (Taxotere) or imatinib (Gleevec) might reduce the chemotherapy drug's effects.
• Certain immunosuppressive drugs. Taking St. John's wort with tacrolimus (Prograf, Astagraf XL, others) or cyclosporine (Neoral, Sandimmune, others) might decrease the drug's effect.
• Certain statins. Taking St. John's wort with simvastatin (Zocor, Flolipid) might reduce the drug's effectiveness.
• Contraceptive drugs. Use of St. John's wort with contraceptive drugs might result in breakthrough bleeding, irregular bleeding or unplanned pregnancy. An additional or alternative form of birth control might be needed.
• Cytochrome P450 1A2 (CYP1A2), Cytochrome P450 2B6 (CYP2B6), Cytochrome P450 2C19 (CYP2C19) and cytochrome P450 2C9 (CYP2C9) and cytochrome P450 3A4 (CYP3A4) substrates. Don't take St. John's wort if you're taking a drug affected by these enzymes.
• Dextromethorphan. Taking St. John's wort with this cough suppressant might increase the risk of the accumulation of high levels of serotonin in your body.
• Digoxin (Lanoxin). Taking St. John's wort with this heart medication can reduce the drug's effect.
• Fexofenadine (Allegra Allergy). Taking St. John's wort with this antihistamine might cause too much of the drug to build up in your body, worsening usual side effects.
• Ketamine (Ketalar). Taking St. John's wort with ketamine might reduce the drug's anesthetic effect.
• Narcotics. Don't take St. John's wort with methadone (Methadose). Taking St. John's wort with certain narcotics might reduce the drug's effectiveness. Combining the supplement with narcotics might also increase narcotic-induced sleep time and painkilling effects.
• Non-nucleoside reverse transcriptase inhibitors. Taking St. John's wort with one of these anti-HIV drugs could reduce the drug's effect.
• Omeprazole (Prilosec). Don't take St. John's wort with this drug used to treat persistent heartburn. The supplement can reduce the drug's effectiveness.
• Phenytoin (Dilantin, Phenytek). Taking St. John's wort with this anticonvulsant might result in loss of seizure control.
• Photosensitizing drugs. Taking St. John's wort with a drug that increases sensitivity to sunlight might increase the risk of a reaction.
• Protease inhibitors. Taking St. John's wort with this type of antiviral drug can reduce the drug's effectiveness.
• Triptans. Don't take St. John's wort with these medications used to treat migraines. The supplement might increase the risk of the accumulation of high levels of serotonin in your body. Too much serotonin can cause mild to severe side effects.
• Voriconazole. Taking St. John's wort with this antifungal drug might reduce the drug's effectiveness.
• Warfarin (Jantoven). Taking St. John's wort with this drug taken to reduce blood clotting might decrease the drug's effect.