||Yes it does. Grapefruit juice (GFJ) has been thought to increase the effects of some drugs for sometime, but only in the 1990's were researchers able to start identifying the specific pharmacology of it.
Grapefruit juice contains a substance which suppresses the action of some enzymes in the body which metabolize some types of drugs. The enzymes CYP3A4 (also called Cytochrome P450 3A4) and CYP3A5 are known to be significantly affected by even small amounts of grapefruit juice in many people.|
An excellent article covering grapefruit juice by Minh Chau Vu is a good introduction to the topic.
Unfortunately there appears to be little research into grapefruit juice's effects on the metabolisis of recreational psychoactives. There are a handful of reports from users who claim that the effects of LSD, Psilocybin, and Ecstasy have been increased by taking grapefruit juice beforehand, but there are also reports from those who say it had no effect. Because of the nature of most underground psychoactives, dosage is difficult to guarantee, and experiences vary naturally and are difficult to accurately compare.
Some SSRI anti-depressants now carry label warnings saying not to take with grapefruit juice and a study from 1999 by Lee et al showed that levels of setraline (Zoloft) were increased when grapefruit juice was taken during a day when the medication was used.
One study, by Maish and colleagues in Pharmacotherapy 1996, looked at whether grapefruit juice increased caffeine effects and found little or no difference in blood concentrations and blood pressure when caffeine was given with and without GFJ. Other research has confirmed these results. Since specific enzyme systems most often metabolize similar chemicals, one would not necessarily expect GFJ to affect other stimulants, but this is far from conclusive.
One psychoactive drug that grapefruit juice is known to affect is valium (diazepam). In a paper by Ozdemir in the European Journal of Drug Metabolism and Pharmacokinetics in 1998, they found that 250ml of grapefruit increased the peak concentration by 50% and increased the time to peak from 1.5 hours to 2 hours.
Added Aug 2008: While grapefruit juice is known to inhibit enzymes involved in the metabolism of codeine and other opiates, I've been unable to find a clear answer about whether codeine's effects are increased or decreased when combined with grapefruit juice. Other opiates seem to be more clearly affected, but PubMed searches result in very few papers that directly address the questions. One paper, by Kharasch et al from 2004 found that grapefruit juice had only a small effect on oral the metabolism of methadone. For codeine, the picture is murky. Discussions at bluelight, drugs-forum.co.uk, and experience reports at Erowid have some reporting that grapefruit juice increases codeine's effects and others reporting it decreases codeine's effects.
Its probably wise to proceed cautiously with intentionally combining strong psychoactives with substances known to inhibit the body's natural metabolism.