Percocet, morphine, oxcycodone..... Need I go on ? All of them are legal for medical uses based upon prescriptions. Heroin is an opiate, just like the forms stated above. The only difference is what's stated above is regulated through the FDA. Why is heroin itself illegal ? It's not regulated by the FDA because it's too readily available compared to the forms of opiates stated above. Those are available as well, just not as easy because those are typically only available under prescriptions.