Is it safe to take phentermine and procardia (nifedipine) together?

No: Phentermine is a raw stimulant which does not burn energy or increase metabolism. It increases heart rate, causes vasoconstriction, increases blood pressure, increases the possibility of cardiac arrhythmia, and causes loss of appetite. It would be dangerous even without a heart-related health problem, so you definitely shouldn't take it if you're on procardia (nifedipine).