Some herbal compounds can increase or potentiate the serum concentrations of certain drugs either by increasing their metabolism to an active form or by slowing down their breakdown. This can sometimes be due to the action of the liver. For example, grapefruit juice contains naringenin, a flavonoid which selectively reduces the Phase 1 metabolism of certain drugs.
In a synergistic reaction you end up with the effect that 1 + 1 = 3.
A naturopathic doctor, or trained professional, will be able to inform you of any synergestic reactions between your herbal supplements and your drugs.