A heroin junky, when high, is quite happy. It's only when they're not fulfilling their desire for drugs that they're not happy.
He experiences a sort of physical/emotional pleasure. However, the habit of taking heroin harms his overall health, and his overall good, so he cannot be truly happy. Happy people do not destroy themselves. In fact, the drug addict is neither happy nor free, but is simply a slave to sin and vice.
It's only when they're not fulfilling their desire for drugs that they're not happy.
In all senses of the word.