Morphine was the first active drug molecule extracted from a plant as well as the first to have analogues prepared from it. The following is a discussion of the history of heroin from Dr. Walter Sneader’s book Drug Discovery: A History. The first analogue was formed by Henry How in 1853 in an attempt to make codeine, another active molecule found in opium poppies, but failed. Later work by Alexander Crum Brown and Thomas Fraser led to the discovery that chemical structure correlated with biological activity, opening the floodgates to pharmacology research. In 1869, Augustus Matthiessen and Charles Alder Wright heated morphine with hydrochloric acid and produced apomorphine, a drug that acts on dopamine D1/D2 receptors and not opioid receptors. Wright continued this work and synthesized many opioid analogues, including diacetylmorphine, i.e. heroin. Ralph Stockman and David Dott tested heroin in frogs and rabbits and found that it was a stronger depressant than morphine, but had weaker narcotic action. Work by E. Merck determined that ethylmorphine, another derivative, was found to work better than codeine so Merck marketed this analogue in 1898, instead. At the same time researchers at Bayer marketed heroin because they found it was superior to codeine as a cough suppressant for turberculosis. Not knowing that it was converted to morphine in the body, the thought at the time was that heroin would be less addictive. A paper in the Journal of the American Medical Association by J. Phillips in 1912 warned of the addictive properties of heroin, which led to legislation banning its use.
Currently, in the U.S., heroin use has been steadily increasing due to a decrease in the use of prescribed opioids like oxycodone and hydrocodone, as shown in the figure below.
This decrease is a result from the fact that it is harder to get these prescribed medications and that heroin is cheaper than these medications. Further, new hard to abuse formulations are not worth the hassle so pure heroin is a better option for most. The problem with this is the fact that heroin is more potent than morphine because heroin passes through the blood brain barrier much easier. As an example, here is a table showing the doses of morphine and heroin given to rats. You can see that a dose of morphine at 120 mg/kg is similar to 8 mg/kg of heroin, at least to produce motor sensitization. It was this paper that really made me think about how different the doses of heroin are compared to morphine.
Anyway, just as a quick conclusion, heroin is more potent than morphine or other opioid compounds. I think the lack of knowledge of this difference may contribute to more overdoses than use of prescription #drugs. Not knowing 1) how much heroin is in your street content and 2) that you need less heroin than oxycodone to get a sufficient high, is a major problem with the new trend in heroin use.