The object of the parent study is to compare the safety and efficacy of dapsone and atovaquone in the prevention of PCP in high risk individuals who are intolerant of trimethoprim and/or sulfonamides therapeutic drug levels are associated with successful prophylaxis. This substudy will evaluate the bioavailability of atovaquone suspension, evaluate population pk parameters and patient outcome and collect/store blood samples for future evaluation of new surrogate marker assays.