Herbal supplements are non-regulated drugs. But they definitely can have effects. Eg. Willow bark tea contains asprin (which can kill young children), Sleepytime tea contains valierian (a sedative), Ma Huang contains pseudophedrine (a weight loss drug that can cause heart attacks)... ginger, garlic, all contain antimicrobial substances.

Check w/your doctor. As Vash said, herbal supplements can lead to a drug interaction if you are on any other medication.