St. John’s Wort has been found to be extremely effective against mild and moderate depression, but its active ingredient, hypericin, reacts with sunlight to produce free radical chemicals which may promote the development of cataracts and nerve damage. Users are advised to wear hats and wrap around sunglasses, and not to use sunbeds or light therapy (e.g. for seasonal affective disorder (SAD).

Cows which graze on St. John’s Wort are known to suffer from extreme photosensitivity, which can prove fatal. The animals develop inflamed and swollen tissue, making swallowing difficult. But there is a silver lining. The herb’s strong reaction with light is being investigated as a potential agent for killing skin cancer cells.