The development of multi-drug resistant strains of HIV remains the primary reason for treatment failure and progression to AIDS in the United States. The failure of a particular multi-drug regimen necessitates a switch to a new multi-drug regimen. We use a simple model of the interaction of resistant strains to show that the transition to the new regimen involves a significant risk of strains resistant to the new regimen emerging, and that treatment interruptions using the failing regimen can be used to decrease this risk.