How does taking St. John's Wort affect blood chemistry?
How does taking herbal remedies such as St. John's Wort affect giving blood? Should you tell the blood donor centre?
I think you should tell them. It's also important that you tell your doctor that you're taking something herbal, because so frequently that doesn't happen. The outcome is that the herbalist doesn't know what drugs you're taking and the doctor doesn't know what the herbalist is giving you.