I've been doing some research and grapefruit, along with other citrus fruits, is actually a urine alkalizer, which keeps the amphetamine in your body for longer.
The following substances are urine alkilizers:
most fruits
most vegetables
Bananas
Chocolate
Figs
Mineral water
Orange juice
Potatoes
Spinach
Watermelon
Dandelion Greens
antibiotics
mineral supplements (including potassium, calcium, iron, magnesium)
antacids
hard water
I also found through testing that I have an acidic body- a pH of 6.0. So I have an acidic body to begin with, so Adderall/amphetamines are not going to be absorbed as well. Alkalizing my urine is the best thing for me.
And that could explain why I felt the grapefruit juice enhancing it. The research I found stated that although citrus fruits are "acidic", they actually basify the body so there should be no problems with medication absorption and in fact just the opposite with citrus fruits.