Decision support systems, as means of disseminating clinical practice guidelines, are powerful software that may lead to an improvement of medical practices. However, they are not always efficient and may suffer from limitations among which are lack of flexibility and weaknesses in the integration of several clinical practice guidelines (CPGs) for the management of patients with multiple chronic disorders. We propose a framework based on an ontological modeling of CPG contents as rules. The ontology provides the required flexibility to adapt patient data and enable the provision of appropriate recommendations expressed at various levels of abstraction. To solve decisional conflicts that occur when combining multiple sources of recommendations, we proposed a method based on the subsumption graph of the patient profiles corresponding to the rules. A prototype CDSS implementing this approach has been developed. Results are given on a clinical case to illustrate the assets of ontological reasoning in increasing the number of issued recommendations and thereby the reliability of decision support.