It's not just the medication NIFEDIAC.  There are a lot of blood pressure medicines that don't mix with grapefruit juice or even eating grapefruit.  Here's one of the reasons.Grapefruit juice contains chemicals that can cause problems with enzymes that break down certain types of medicines in your intestines. When a medicine does not get broken down properly in the intestines, you can end up with too much medicine in your blood. Having too much medicine in your blood increases your chances of having serious side effects.  One is that it can cause very low blood pressure.  Hope this helps.  Good luck.