Hyperforin is the one most commonly attributed with giving St John s Wort its ability to act as an antidepressant. Hyperforin inhibits the reuptake of serotonin, norepinephrine and dopamine in the brain allowing them to stay at higher levels in the brain rather than being reabsorbed. These are thankfully pretty rare. However, if you do develop any side-effect after taking St John s Wort, stop taking it immediately and if in any doubt seek medical help. The side effect may be due to the dosage or the particular St John s Wort product you have taken.
Some people develop a rash.
This could be an allergy to the herb itself. However as most people take St John s Wort in tablet or capsule form rather than as an infusion made from the actual herb it could be a reaction to something else involved in its manufacture. Like any new substance introduced to the body for the first time, you could also suffer an upset stomach as a result of taking St John s Wort. Another potential side-effect is increased photo sensitivity. This means you become more sensitive to sunlight and have to be even more aware of the need for sunscreens than most people.