▁The ▁human ▁car cin ogen icity ▁of ▁phen obar b ital ▁has ▁been ▁the ▁subject ▁of ▁debate , ▁but ▁it ▁is ▁currently ▁class ified ▁in ▁group   2 B ▁by ▁the ▁I AR C ▁and ▁this ▁appears ▁to ▁have ▁been ▁an ▁important ▁consideration ▁in ▁the ▁classification ▁of ▁mus k ▁x yl ene ▁as ▁a ▁category   3 ▁car cin ogen ▁under ▁the ▁D anger ous ▁Sub st ances ▁Direct ive . ▁Nevertheless , ▁the ▁European ▁Union ▁R isk ▁Ass ess ment ▁Report ▁adm its ▁that ▁mus k ▁x yl ene ▁is ▁a ▁" border line ▁case ". ▁ ▁A ▁further ▁comp lication ▁is ▁the ▁met abol ism ▁of ▁mus k ▁x yl ene . ▁One ▁route ▁of ▁met abol ism ▁is ▁through ▁reduction ▁of ▁one ▁or ▁more ▁nit ro ▁groups ▁by ▁the ▁int est inal ▁micro fl ora ▁( g ut ▁b acter ia ) ▁to ▁produce ▁a rom atic ▁am ines ▁such ▁as ▁p - N H 2 - mus k ▁x yl ene . ▁This ▁met abol ite ▁has ▁a ▁different ▁li ver ▁to x icity : ▁in ▁particular , ▁it ▁in hib its ▁the ▁C Y P 1 B ▁en zym es ▁by ▁c oval ent ▁binding . ▁ ▁Ind u ction ▁of ▁cy to chrome ▁P 4 5 0 ▁en zym es , ▁the ▁most ▁likely ▁cause ▁of ▁rod ent ▁car cin ogen icity , ▁is ▁a ▁threshold ▁phenomen on , ▁with ▁a ▁no ▁observed ▁effect ▁level ▁( NO EL ) ▁of ▁ 1 0   m g /