I'm a toxicologist. This is an area I understand very well.

You've asked the important question. Yes, it is possible to take less of some drugs, if you take them with grapefruit juice. In fact, if you naively used grapefruit juice during the period when you were increasing the dose of a new drug up to the comfortable and effective dose (the dose titration period), you may have taken advantage of that effect from the beginning.

A friend of mine was using the mood-stabilizer Tegretol (carbamazapine), which is metabolized by this particular enzyme, 3A4. He was also in the habit of drinking about sixteen ounces of grapefruit juice every morning. When he found out about the grapefruit/carbamazapine interaction, he was going to suddenly stop the grapefruit juice, but I talked him out of it. If he had done so, he would have had to increase his carbamazapine dose to compensate, and, there was no way of predicting how much more drug he would have to take. He'd have to do a completely new dose titration under these new conditions. As it was, he had titrated his dose with grapefruit juice inhibition already accounted for (it was a daily ritual), so all he had to do was keep doing what he had always done, to maintain the proper therapeutic dose.

Grapefruit is not automatically a no-no. It certainly must be taken into account. But it depends on a number of factors, and the individual circumstances.

Lar