In addition to drug interactions, the following influence activity of some antiretroviral drugs and should be avoided: Grapefruit juice, which inhibits an enzyme in the gastrointestinal tract that degrades the PI saquinavir and thus increases bioavailability of saquinavir St John's wort, which can enhance metabolism of PIs and NNRTIs and thus decrease plasma PI and NNRTI levels Adverse effects of antiretrovirals Antiretrovirals can have serious adverse effects.