Introduction:
Microorganisms should be removed from drugs during their production, however sterilization processes should not produce free radicals. Free radicals in drotaverine were studied. Usefulness of thermal sterilization for drotaverine was checked.

Materials and methods:
Drotaverine heated at 160 oC was examined by EPR method. Free radical concentration, their distribution in the sample, complex character of free radical system and magnetic spin-spin interactions in drug were determined. Changes of free radical concentration in drotaverine during storage after sterilization were examined.

Results:
EPR lines were not obtained for drotaverine before sterilization. It was pointed out that drotaverine after thermal sterilization at temperature 160 oC contains free radicals and reveals EPR spectra. Free radical concentration in drotaverine changes with storage time after thermal treatment. Interactions with oxygen are probably responsible for this effect. Free radicals are homogeneously distributed in drotaverine. Free radicals are nearly located in drotaverine and they strongly dipolar interact, what cause broadening of EPR spectra. Continuous microwave saturation of EPR spectra bring to light slow spin-lattice relaxation processes in thermally sterilized drotaverine. Shape and asymmetry parameters of EPR spectra of the tested drug change with increasing of microwave power, what indicate complex character of free radical system in the sample.

Conclusions:
Drotaverine should not be thermally sterilized, because of free radical formation. The obtained results confirmed usefulness of EPR spectroscopy for examination of thermally sterilized drugs. EPR method may be used as the additional one to microbiological tests.