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St John's wort is widely known as a herbal treatment for depression. In some countries, such as Germany, it is commonly prescribed for mild depression, especially in children and adolescents.
 A report in the Cochrane Review states,
The available evidence suggests that the hypericum extracts tested in the included trials a) are superior to placebo in patients with major depression; b) are similarly effective as standard antidepressants; and c) have fewer side effects than standard antidepressants. There are two issues which complicate the interpretation of our findings: 1) While the influence of precision on study results in placebo-controlled trials is less pronounced in this updated version of our review compared to the previous version (Linde 2005a), results from more precise trials still show smaller effects over placebo than less precise trials. 2) Results from German-language countries are considerably more favourable for hypericum than trials from other countries.
Standardized extracts are generally available over the counter, though in some countries (such as the Republic of Ireland) a prescription is required. Extracts are usually in tablet or capsule form, and also in teabags and tinctures. Herbalists are more likely to use a fluid extract than a tincture. Hypericum was prescribed in ancient Greece, and it has been used ever since.
St. John's wort is being studied for effectiveness in the treatment of certain somatoform disorders. Results from the initial studies are mixed and still inconclusive; some research has found no effectiveness, other research has found a slight lightening of symptoms. Further study is needed and is being performed.
A constituent chemical, hyperforin, may be useful for treatment of alcoholism, although dosage, safety and efficacy have not been studied. Hyperforin has also been found to have antibacterial properties against gram-negative bacteria, although dosage, safety and efficacy has not been studied.
A randomized controlled trial of St. John's wort found no significant difference between it and placebo in the management of ADHD symptoms over eight weeks. However, the St. John's Wort extract used in the study, originally confirmed to contain 0.3% hypericin, was allowed to degrade to levels of 0.13% hypericin and 0.14% hyperforin. Given that the level of hyperforin was not ascertained at the beginning of the study, and levels of both hyperforin and hypericin were well below that used in other studies, little can be determined based on this study alone.
A research team from the Universidad Complutense de Madrid (UCM) published a study entitled, “Hypericum perforatum. Possible option against Parkinson's disease”, which suggests that this plant with antidepressant properties has antioxidant active ingredients that could help reduce the neuronal degeneration caused by the disease.
Recent evidence suggests that daily treatment with St. Johns wort may improve the most common physical and behavioural symptoms associated with premenstrual syndrome.
Externally, St.-John's-wort oil is used for the treatment of wounds, abrasions, and firstdegree burns. 
 Adverse effects and drug interactions
St John's wort is generally well tolerated, with an adverse effect profile similar to placebo. The most common adverse effects reported are gastrointestinal symptoms, dizziness, confusion, tiredness and sedation.
St John's wort may rarely cause photosensitivity. This can lead to visual sensitivity to light and to sunburns in situations that would not normally cause them. Related to this, recent studies concluded that the extract reacts with light, both visible and ultraviolet, to produce free radicals, molecules that can damage the cells of the body. These can react with vital proteins in the eye which, if damaged, precipitate out causing cataracts.