The theoretical effects of adherence to protease inhibitors in HIV-1 treatment are investigated. Impulsive differential equations are used to develop a mathematical model of protease inhibitor monotherapy. A drug concentration threshold that defines when drug resistance will emerge is determined. The following research questions are addressed: a) how many doses may be missed before resistance emerges? b) How many subsequent doses must be taken to return to pre-interruption drug levels? Estimates are provided for all protease inhibitors approved by the US Food and Drug Administration. The basic reproductive ratios for the wild-type and mutant strain of the virus are calculated, for a long-term average fractional degree of adherence.