The invention relates to the use of active ingredients, which increase the concentration of pyrimidine-based elements for nucleic acid biosynthesis in the body, in particular to the use of pyrimidine nucleosides and/or prodrugs produced therefrom, for reducing the side-effects of inhibitors of nucleic acid biosynthesis or their precursors, in particular by activating the biosynthesis of mitochondrial DNA (mtDNA). The invention also relates to the use of said active ingredients, in particular pyrimidine nucleosides and/or prodrugs for producing pharmaceutical preparations for reducing the aforementioned side-effects and to combinations or products for administering active ingredients of this type, in particular pyrimidine nucleosides and/or prodrugs produced therefrom, comprising inhibitors of nucleic acid biosynthesis or their precursors. The invention further relates to methods for treating the side-effects of inhibitors of nucleic acid biosynthesis or their precursors using the aforementioned active ingredients, in particular pyrimidine nucleosides and/or prodrugs produced therefrom, or the aforementioned combinations or products and to corresponding pharmaceutical preparations. Side-effects of HAART (Highly Active Anti-Retroviral Therapy) and side-effects of other anti-viral nucleoside analogous agents, which inhibit the mitochondrial -polymerases, can in particular be prophylactically and/or therapeutically treated in this manner.