Stolen from www.mesomorphosis.com who got it from medline:

Just got off Medline and read about how important CYP3A is in metabolizing several substances, from Diazepam to Anabolic Steroids...

CYP3A, which is dramatically reduced by consumption of Grape Fruit Juice, is needed for metabolism of all steroids - testosterone and cortisol. Consuming Grape Fruit Juice has been proven to lower CYP3A levels and thus allows for a much slower metabolism of oral and injectible testosterones, allowing you to benefit more from the drug since lower CYP3A levels means more of the drug active in your system.

It is clear that by taking Grape Fruit Juice, especially with oral anabolics, that it will lower CYP3A levels and thus metabolize oral AAS at a much slower rate, allowing for more of the oral anabolic into your system and hence allowing for more muscle building effects to occur with your dose of aas. Good thing to think about next time you pop a valium, dianabol, or anadrol-50. It Also plays a role in metabolizing substances which I WILL NOT mention on this board.

As well as Grape Fruit Juice reducing CYP3A, so does the herb MILK THISTLE. So much for the theory that taking MILK THISTLE with your oral AAS will interfere with oral anabolic absorbtion. In fact, taking MILK THISTLE with oral anabolics should POTENTIATE the effects of oral anabolics in your system by a very large degree.

However, there are herbs, like St. John's Wort for instance, which greatly increase CYP3A levels, which will thus greatly increase the metabolism of anabolics in your system and thus will NOT allow for longer action of oral anabolics in your system.

Seems like a good way to slash CYP3A would be to take some grape fruit juice with some milk thistle and then take your oral aas.