Terbinafine is not an inducer or inhibitor of CYP3A4 and other major CYP enzymes in vivo.Risk for gastrointestinal adverse events in the form of abdominal distension, reduced appetite, nausea, dyspepsia, mild abdominal pain and diarrhoea motivates vigilance against insufficient intake of food, especially of carbohydrate.

Dimethylhept-e-yn (methyl)- (naphtyl methyl)- amine M=291.

Terbinafine is indicated for the treatment of fungal infections of the skin and nails. Very common side effects that can be potentially porphyrinogenic through reduction in carbohydrate intake and that also can be confused with an acute porphyria attack are abdominal distension, reduced appetite, nausea, dyspepsia, mild abdominal pain and diarrhoea. Other common side effects are arthralgia and myalgia.

Terbinafine is metabolized by CYP2C9, CYP1A2, CYP3A4, CYP2C8 and CYP2C19. The half-life elimination is between 17-400 hours (Castberg 2005). In vitro and in vivo data indicates that terbinafine is a competitive inhibitor of CYP2D6 (Castberg 2005 and Vickers 1999). In vitro studies have shown that terbinafine had little or no effect on theophylline (CYP1A2), tolbutamine (CYP2C9), (S)-mephenytoin (CYP2C19) and cyclosporine A (CYP3A). The potential for drug-drug interactions with CYP1A2, CYP2C9, CYP2C19 and CYP3A is predicted to be insignificant (Vickers 1999). A clinical study showed that co-administration of terbinafine and midazolam, a CYP3A4 substrate, did not decrease the AUC of midazolam significantly (Ahonen 1995). This indicates that terbinafine is not an inhibitor or an inducer of CYP3A4. While a clinical study showed that the AUC of cyclosporine (CYP3A substrate) decreased by 12.9% when co-administered with terbinafine (Long 1994), another showed that co-administration of terbinafine and theophylline (CYP3A4 substrate) resulted in an increase of the AUC by 16% (Trépanier 1998). Both the decrease and increase in AUC is however, according to the U.S Food and Drug Administration, too low for terbinafine to be considered as an inducer or an inhibitor (FDA).