ddI (didanosine, Videx / VidexEC) is a common component of anti-HIV drug combinations. It should be taken with at least two other anti-HIV drugs to suppress HIV to low levels.

Approval of ddI was based on the results of four non-randomised studies into ddI taken alone. In all four studies, patients taking ddI alone showed greater increases in CD4 cell counts than patients from historical studies receiving no HIV treatment^[1][2][3][4]. This led to the drug’s approval in October 1991, being the second antiretroviral drug to be licensed, after AZT (zidovudine, Retrovir).

Further studies compared ddI monotherapy to AZT monotherapy, and the effects of switching from AZT to ddI monotherapy on disease outcomes. In general, these found that AZT and ddI were of similar efficacy in terms of disease progression and survival in patients with little prior AZT experience^[5][6].

Switching from AZT to ddI after at least four months’ treatment was also found to be beneficial, resulting in less disease progression and death than remaining on AZT monotherapy, even in patients with substantial resistance to AZT before switching^{[7][8][9][10][11][12][13][14]}. ddI was also found to be beneficial in patients who were intolerant to AZT^[15][16][17].

Later studies of ddI examined the effects of adding the drug to AZT monotherapy, in patients with and without experience of taking AZT. These trials, including Delta, ACTG 175, NuCombo and BW 34,225-02, found that the combination of AZT and ddI was associated with better survival and less disease progression than AZT taken alone, particularly in patients who had not taken AZT before^{[18][19][20][21][22]}. This was related to greater decreases in viral load and increases in CD4 cell counts in the patients taking AZT and ddI^{[23][24][25][26]}.

Several studies have found that adding hydroxycarbamide (Hydrea), previously known as hydroxyurea, to ddI-containing regimens increases the drug’s antiviral effects^[27]. However, hydroxycarbamide can dampen down the CD4 cell count rise and cause significant side-effects. For more information, see Hydroxycarbamide (Hydrea).

ddI reaches high levels in semen, where it may have anti-HIV activity^[28].