NRTIs block an enzyme of the HIV called reverse transcriptase that allows HIV to infect human cells, particularly CD4 T cells or lymphocytes. Reverse transcriptase converts HIV genetic material, which is RNA, into human genetic material, which is DNA. The human-like DNA of HIV then becomes part of the infected person's own cells, allowing the cell to produce RNA copies of the HIV that can then go on to attack other not yet infected cells. Thus, blocking reverse transcriptase prevents HIV from taking over (infecting) human cells.
In general, most antiviral regimens for HIV disease contain a backbone of at least two NRTIs. The NRTIs include ZDV, d4T, ddI, zalcitabine (HIVID, ddC), 3TC, FTC, abacavir (Ziagen, ABC) or TDF. The NRTIs FTC and 3TC are highly related compounds and, although data is somewhat limited, most experts agree that they probably can be used interchangeably. That said, many combinations of NRTIs can be used together, with current guidelines generally recommending the fixed-dose combination of TDF with FTC with alternatives being the fixed-dose combinations of ABC/3TC or ZDV/3TC. Other options would include ddI plus 3TC or FTC. ABC has been associated with severe allergic reaction in approximately 5% of patients. Recent studies have shown that a blood test can be performed to determine who is at risk for this reaction so that the drug can be avoided in these individuals and be used in others with greater confidence that there will not be such a reaction.