A biocidally active polymer composition formed from an oligomeric or polymeric additive and a thermoplastic polyurethane polymer is disclosed. The biocidally active polymer may further include a non-fouling and/or non-thrombogenic polymer. The biocidally active polymer provides biocidally active properties, and may also provide non-fouling and/or non-thrombogenic properties. The composition is particularly useful in articles and materials for medical applications.