5-Nitrofurfural (NFA) an important photodecomposition product and metabolite of medicinal nitrofurans is phototoxic in bacterial test systems. Its major photodecomposition product 5-hydroxymethylene-2(5H)-furanone (HMF) appears to be responsible for this. Furthermore HMF and photoactivated nitrofurfural can induce repairable DNA damage in Escherichia coli bacteria. These effects may be important with respect to skin and allergic reactions and carcinogenicity appearing in nitrofuran therapy.