ST. JOHNS WORT St. John's Wort is one of the oldest known herbal remedies. It has been used for over 2000 years to treat emotional and nervous complaints. It grows throughout Europe, Western Asia and North Africa. The active substance of St. John's Wort is Hypericine. Clinical studies have shown that St. John's Wort may enhance mood and promote a healthy sense of well being, and it is for these properties that it is widely taken. John's Wort can negatively influence the effect of certain medicines. It enhances the effect of the liver enzyme cytochrome P450. This causes some medicines to be broken down faster, making them less effective. When combining a medicine with St. John's Wort you are advised to consult your physician or your thrombosis prevention unit for information whether the combination can lead to such an interaction. It can also influence the effect of SSRI-typeantidepressants. The combination is discouraged.