Morpheus is the Greek god of dreams. It is hardly surprising then that he lent his name to morphine, a bringer of dreams.
In the nineteenth century opiates were wildly popular. Morphine, the active ingredient in opium, was used in cough medicines. It was used in laudanum, which was routinely used to soothe crying babies. It was used for recreation. Everyone was taking drugs. They were wildly popular. In fact, they were so popular, England fought wars for the sole and altruistic purpose of allowing other peoples to have access to their wonderful benefits.
Thomas De Quincey is, perhaps, the best known of these nineteenth century drug addicts. This is doubtless due to his book, Confessions of an English Opium Eater; a memoir that he knocked out, as he was short of funds for his opium habit. The book is quite fantastic, full of strange and awesome dreams, doubtless doing its sales no harm at all, which funded his habit until his death at the ripe old age of seventy-four. By which time he had been taking opium for fifty-five years.
However, as with all wonderful things, some people started to worry and felt that all this drug taking might be addictive, that it might have negative consequences; well, that is what some people said, but we may suspect they were afraid if people were too happy they might be less inclined to be religious, or, even worse, they might not wish to work any harder or more frequently than they needed.
Anyhow, a pharmaceutical company by the name of Bayer, at the end of the nineteenth century, decided to try to create a non-addictive alternative to morphine. And they invented diacetylmorphine. Everything was great, except the name. Customers could not be expected to ask for diacetylmorphine. So they gave people their product and asked them how they felt. They all said they felt wonderful or words to that effect; however, one word in particular came up over and over again. The word was hero. People said it made them feel like a hero. So the clever people at Bayer decided to call their new product: hero-in. And that is how a Greek god was overthrown by a hero.
Oh, by the way, it seems the clever people at Bayer may have been a mite over optimistic in their belief that heroin would be non-addictive.