Rash and liver toxicity can occur on either nevirapine or efavirenz. Rash and liver toxicity is generally less severe on efavirenz. Cases of hypersensitivity reaction, a life-threatening syndrome of rash, fever, abdominal pain, diarrhoea, dry cough and jaundice, have been reported in only a few patients on efavirenz.ash and liver toxicity can occur on either nevirapine or efavirenz. Rash and liver toxicity is generally less severe on efavirenz. Cases of hypersensitivity reaction, a life-threatening syndrome of rash, fever, abdominal pain, diarrhoea, dry cough and jaundice, have been reported in only a few patients on efavirenz.