ENF is injected into the abdomen, deltoid, or the anterior aspect of the thigh. Patients will be taught how to self-administer ENF. Medical staff will observe self-injection of the first dose of ENF and at clinic visits scheduled for Weeks 1, 2, and 4. During Week 4, patients will undergo pharmacokinetic testing. This requires that patients come to the clinic for approximately 12 hours so that blood can be tested at different times after taking the study drugs.