paternalism

John Stuart Mill though that it was only permissable for the state to restrict someones liberty to prevent them from causing harm to others. Why? is Mill correct? Are paternalistic policies ever justifiable? If not, why not? if so, when are they justified? Even Mill thought that it would be permissable to forcibly prevent the foreign traveler from crossing the broken bridge. Would the state be justified in prohibiting the liberty of heroin users for their own sake? Are drug users like the foreign traveler?