tolerance

[iNsom]

I've heard that grapefruit juice contains a certain enzyme that potentiate cns depressants.

Hey,

mmm, kind of, ish.

Grapefruit juice actually inhibits / blocks enzyme(s) found in the intestine which are responsible for metabolising certain meds (diazepam for example) - thus more enters the blood (potentiated) 
This can be dangerous especially if someone is taking st@tins (for example) 

On the other hand some medications require this enzyme (codeine for example) and in this case grapefruit will actually reduce the effects. 

The list of drugs grapefruit potentiates or reduces effectiveness is huge.