Having an unexpected/unwanted child sure can, as can STI's like HIV. My point, which I hoped was clear, is that abstinence (whether from drugs, sex, or anything else) is only a workable model at the societal level in theory. In practice we already know that a statistically significant number of people will engage in risky behavior. Harm reduction in the context of sex involves early education, access to condoms, etc. Harm reduction in the context of drugs involves early education, access to quality rehab and mental health services, safe injection sites, etc.