As others already answered, مدمن = addict. مدمناً = مدمن it's the same word, the "اً" at the end is something called "tanween" "تنوين" , I don't know if you study i'rab or not yet, but anyway, that addition is only grammatical, it doesn't affect the meaning. مدمن = "mudmin" is the active participle of the verb "أدمن" (root: دمن).

"مدمن"=it's an adjective ,it means "addicted" مدمن الشرب=addicted to alcohol=alcoholic "مدمن الشرب" &" سكير" mean the same

addict / 상용자

Addicted to