's effects directly have nothing to do with dental decay, but poor food intake (specially the lack of fruit and greens) along with bad hygiene, opens room for problems. Street addicts, who live in bad conditions, are prone to those problems. When their bodies are invaded by oportunistic bacteria and viruses - specially of those who use needles - overall oral health tends to suffer as well. With time, the problem only gets worse.
The regular use of heroin, by itself, is not the cause of those problems.