In economics, a demerit good is "a good or service whose consumption is considered unhealthy, degrading, or otherwise socially undesirable due to the perceived negative effects on the consumers themselves";[1][2][3] it could be over-consumed if left to market forces of supply and demand.
Examples of demerit goods include tobacco, alcoholic beverages, recreational drugs, gambling and junk food.
[4] There is an important conceptual distinction between a demerit good and a negative externality.
By contrast, a demerit good is considered as undesirable because its consumption has negative effects upon the consumer.
Paternalism judges that heroin "isn't good for you", and feels free to override the judgement of the addicts themselves.