A person addicted to drugs; a straightforward compound of 麻薬 (narcotic) and 患者 (patient).
He is receiving treatment as a drug addict.
中毒者 is a broader term for an addict or someone suffering from poisoning, not limited to drugs.
薬物依存症 refers to drug dependence as a medical condition, whereas 麻薬患者 emphasizes the person as a patient.
Compound of 麻薬 (narcotic, drug) and 患者 (patient).