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 . 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 and side - effects of other anti - viral nucleoside analogous agents , which inhibit the mitochondrial γ - polymerases , can in particular be prophylactically and / or therapeutically treated in this manner .