The objective of the study was to determine if the co-administration of loperamide (LOP) with tipranavir (TPV), ritonavir (RTV), or TPV plus RTV (TPV/r) caused a clinically significant change in the respiratory response to carbon dioxide (CO2), defined as a 10% decrease in the area under the pharmacodynamic effect/time curve or at least a 25% decrease in at least one pharmacodynamic time point.