Soulja Boy says it's wrong to take high-end cough syrup off the market ... because thousands of sick people need it -- and he REALLY loves it -- so he's on a mission to bring Actavis back.

Soulja Boy tells TMZ ... he's "mad as hell" over Actavis' decision to cease all production of promethazine codeine -- saying it doesn't punish rappers -- it hurts the infirmed.

S.B. doesn't think Actavis' decision had anything to do with celebrities drinking it -- even though that's what the company said, adding, "Actavis is running away from money ... they could be selling out everywhere."

Soulja says he drinks Actavis on the road so he can sleep -- and he says there's nothing wrong with that ... so he's rallying his friends together in a petition drive.

Soulja adds, "As far as the lean goes, just know I been had the juice since 2011."

Wild guess ... that testimonial won't help his cause.