You also have to worry about 1st Pass Liver metabolism. Something that interferes with the CYP Enzymes can leave more of the active compound available. That's where Grape Fruit Juice, Berberine, or Cimetadine (OTC Tagamet HP) come in. But, there can be downsides to CYP inhibitors too, so read up. Your liver works harder, and I know Tagamet had some reports of DHT/Estrogen interference leading to some problems in men (See Wikipedia).