Why should you avoid Grapefruit juice with Flexeril ?

From https://www.drugs.com/flexeril.html

Eating grapefruit or drinking grapefruit juice may increase the risk of Flexeril 's side effects. Talk with your doctor before including grapefruit or grapefruit juice in your diet.

Bob,
I'm not sure why it does, but I do know that grapefruit juice does some odd things with other medications as well. For instance, it can increase the effects and side effects of Xanax. It may do the same for flexeril or it may decrease the drugs effects. Hope this helped somewhat.