People who are infected with HIV do not have the symptoms usually associated with the diagnosis of full-blown AIDS. When HIV-infected people are diagnosed with AIDS, they develop one or more of the following major complications: life threatening infections, cancers, and neurological problems.

One of the infections is Pneumocystis carnii pneumonia (PCP). This form of pneumonia is the major AIDS define diagnosis for HIV infected patients. In addition, PCP is usually the identifiable cause of death in most patients with AIDS. The average CD4 count for the first episode of PCP in an infected individual is about 100/mm³. The risk of PCP for patients with a CD4 cell count less than 100/mm³ are 40-50% per year; however, the percentage rises to 60-70% per year for patients with a prior PCP history. This disease is considered to be clearly the most common serious complication of HIV.

Bactrim^TM is a drug that provides protection against PCP in individuals that are infected with the virus and a considerable high risk of developing pneumonia. Bactrim^TM was designed by Roche Laboratories and has shown a considerable reduction of the PCP infection. It is a 5:1 ratio of sulfamethoxazole(SMZ) and trimethoprim(TMP). SMZ inhibits bacterial synthesis of dihydrofolic acid by competing with para-aminobenzoic acid. TMP blocks the production of tetrahydrofolic acid from dihydrofolic acid by binding to and reversibly inhibiting the required enzyme, dihydrofolate reductase. Bactrim^TM therefore, blocks two consecutive steps in the biosynthesis of nucleic acid and proteins essential to the PCP bacteria.

Bactrim^TMis given to HIV-infected patients who are susceptible to PCP including those with prior PCP diagnosis, a CD4 cell count less than 200/mm^3, HIV-associated thrush, and unexplained fevers above 100 degrees Fahrenheit for more than two weeks. Bactrim^TM is recommended to be prescribed for the treatment of PCP at a dosage of 75-100 mg/kg of SMZ and 15-20 mg/kg of TMP every 24 hours for 14-21 days. This dosage is taken in equally divided proportions once every six hours. The recommended dosage for the prevention of PCP is one double-strength tablet containing 160 mg TMP and 800 mg SMZ. However, the frequency of adverse reactions is high, so the dosage is often reduced if the reaction was not considered severe enough.