AZT (zidovudine, Retrovir) is the only drug whose effectiveness has been shown in reducing HIV transmissions in human studies, in a group of healthcare workers receiving AZT monotherapy after needlestick injury^[1]. Because of this evidence, AZT is often recommended for use in first-line PEP regimens. However, other nucleoside and nucleotide reverse transcriptase inhibitors (NRTIs / NtRTIs), such as tenofovir (Viread) may also be effective. Indeed, some experts believe that tenofovir is better tolerated than AZT.

The only NRTI that is not recommended for PEP is abacavir (Ziagen). Around 8% of patients with established HIV infection develop a severe allergic reaction when they start taking abacavir. Although a genetic test is available in some clinics that can predict whether a patient is likely to experience this reaction, the availability of other drugs means that abacavir is best avoided as prophylaxis.

Side-effects from NRTIs and NtRTIs often include vomiting and nausea. Anti-sickness tablets can be prescribed long with these drugs to reduce these side-effects. Other side-effects include feeling unwell, diarrhoea and headache. These are usually mild, meaning that NRTI / NtRTI combinations tend to benefit from fewer side-effects than combinations containing protease inhibitors or non-nucleoside reverse transcriptase inhibitors (NNRTIs). Although large-scale trials comparing the effectiveness of these regimens in preventing HIV infection have not been carried out, a study comparing a combination of AZT, 3TC (lamivudine, Epivir) and d4T (stavudine, Zerit) following non-occupational exposure found lower rates of side-effects than those given AZT 3TC and nelfinavir (Viracept). This resulted in fewer discontinuations before the end of the four-week treatment^[2].