constant user; addict
Refers to someone who uses something habitually or is addicted to it, often with a negative connotation.
He is a habitual smoker.
Constant users of this medicine need to be careful about side effects.
中毒者 is a stronger term for 'addict', often implying physical or psychological dependence.
Compound of 常用 (じょうよう, 'habitual use') and 者 (しゃ, 'person').