In study 1 , mean serum total testosterone concentrations in healthy male volunteers were similar at baseline for all treatment periods ; 6 hours after dosing , mean concentrations were comparable between placebo ( 8.6 nmol/L ) and tapentadol immediate release ( IR ; 43 mg , 8.8 nmol/L ; 86 mg , 9.3 nmol/L ) , but were lower following administration of morphine IR 30 mg ( 5.4 nmol/L ) .