Fr Peter McVerry says the homelessness crisis means drug users aren't given the chance to rehabilitate themselves.
The campaigner's been speaking in the wake of the shooting dead of Martin O'Rourke yesterday, who was killed in a case of mistaken identity.
Fr. McVerry, who worked with the 24 year old, said he had tried to get clean, but suffered a relapse.
And he says the housing crisis makes it impossible for those trying to recover.
"They leave the treatment programme, they've put a huge amount of work into themselves, and if they look for accommodation they will be placed in a dormitory full of drug users."
"You won't survive in a dormitory full of drug users for more than two or three days without relapsing back into drugs. We really are making it extremely difficult for homeless people to overcome their addiction and we're really putting it up to them and saying 'well why do you bother trying, because you're not going to succeed.'"