This paper, lays down the logical foundations for a personalized medical prescription system. The proposed system employs detailed pharmaceutical and medical knowledge about a medication and its effects or side effects on the patient, which may go beyond what is available in medication leaflets. The ontology was initially built for the proposed system and employs the description logic system, ALC, for knowledge representation. However, the uncertain nature of medical and medicinal knowledge poses some problems such as drug-drug interactions and drug-disease interactions, which render ALC inadequate to represent and reason within a system such as a personalized medical prescription system. Indeed, there is a need for a more flexible representation that allows reasoning with incomplete knowledge and possibly inconsistent cases. ALC is extended with defeasible rules to obtain defeasible ALC. Defeasible ALC allows the prevention of adverse drug interactions, detection drug-drug interactions and detection of drug-disease interactions. The ultimate purpose of this paper is providing to provide a standard knowledge base system toward a medical prescription capable of dealing with incomplete knowledge, conflicting information (inconsistencies) and exceptions cases, which will enhance individual healthcare and provide an appropriate prescription. This is accomplished by expanding the capabilities of description logic with defeasible rules, to achieve an accurate prescription decision for any patient's condition(s). Once implemented, a personalized medical prescription system intends to assist, not to replace, the clinician during medical prescription(s) or the pharmacist(s).