St John's wort
The flowering tops of St. John's wort (Hypericum perforatum) are used to prepare concentrated extracts. The active ingredients are thought to be hypericin and hyperforin, but this is difficult to be certain about as the herb contains several organic compounds. As there is no standardisation of herbal medicines production, St John's wort is usually sold by hypericin content, though the proportion of other ingredients is often not stated. The lack of a clear active ingredient makes studying the pharmacology difficult, but it is thought to act in a mechanism similar to SSRIs.
A Cochrane Systematic Review  reported that the current evidence was “inconsistent and confusing”
The American National Center for Complementary and Alternative Medicine (part of the National Institute for Health) has a fact sheet on St John's Wort . They report that there is some evidence for the use St. John’s Wort is useful for treating mild to moderate depression. However, two large studies (one sponsored by them), have shown no effectiveness in major depression of moderate severity.
See also: Evidence-Based Mental Health “Review: St John’s Wort may be less effective than previously thought in people with depression” 
Taken from the National Center for Complementary and Alternative Medicine fact sheet 
St. John's wort can cause increased sensitivity to sunlight. Other side effects can include dry mouth, anxiety, dizziness, gastrointestinal symptoms, fatigue, headache, or sexual dysfunction.
The herb affects the way the body processes or breaks down many drugs; in some cases, it may speed or slow a drug's breakdown. Drugs that can be affected include:
- Indinavir and possibly other drugs used to control HIV infection.
- Irinotecan and possibly other drugs used to treat cancer
- Ciclosporin, which prevents the body from rejecting transplanted organs
- Digoxin, which strengthens heart muscle contractions
- Warfarin and related anticoagulants
- Birth control pills
When combined with some antidepressants, St. John's wort can increase side effects.
- ↑ Cochrane Collaboration. St John's Wort for depression. 2005
- ↑ NCCAM. St. John's Wort
- ↑ Hypericum Depression Trial Study Group. Effect of Hypericum perforatum (St John's wort) in major depressive disorder: a randomized controlled trial. JAMA. 2002 Apr 10;287(14):1807-14.
- ↑ Evidence-Based Mental Health. Review: St John’s Wort may be less effective than previously thought in people with depression. 2004
- ↑ NCCAM. St. John's Wort
- ↑ Ernst E, Rand JI, Barnes J, Stevinson C (1998). Adverse effects profile of the herbal antidepressant St. John's wort (Hypericum perforatum L.). Eur J Clin Pharmacol 54 (8), 589-94.
- ↑ Piscitelli SC, Burstein AH, Chaitt D, Alfaro RM, Falloon J. Indinavir concentrations and St John's wort. Lancet. 2000;355:547-8.