But apparently the effect lasts up to 12 hours, vitamin c should be gone from the stomach by then. Grapefruit has been anecdotally recommended for MDMA (an amphetamine).
The reason it's recommended for MDMA is because it inhibits the CYP3A4 enzyme which metabolizes MDMA (3,4-Methylenedioxymethamphetamine) to MDA (3,4-Methylenedioxyamphetamine). MDA is more neurotoxic than MDMA which is why it's advised to reduce the amount your body intakes.