Ozonation was studied as a pretreatment method for a waste biological sludge, contaminated with large amounts of globally used antibiotics; tiamulin, levofloxacin and amoxicillin. At the concentrations of over 100 mg L-1, these antibiotics were found to inhibit biogas production (up to 50%), which means, that the molecules would pass the anaerobic digestion system unchanged. Ozonation of sludge, contaminated with antibiotics, not only removes inhibitory effects but also improves overall biogas production for 10% or even more, depending on the dose of the oxidant.