Co-administration of alovudine and zidovudine at ratios considerably in excess of the prior art completely or substantially abolishes the mitochondrial toxicity of alovudine in mitochondrial DNA depletion experiments. The invention thus provides pharmaceutical compositions comprising alovudine and zidovudine in a molar ratio in the range 1:100 to 1:350 and methods for the treatment or prophylaxis of multiply resistant HIV comprising the simultaneous or consequential administration of alovudine and zidovudine in the characteristic molar ratio.
