At the risk of self-promotion, here's an excerpt from my book on opiates and painkillers (currently sitting on my editor's desk):
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In the 1950s heroin addicts in Hong Kong began using a new method of ingesting their heroin. Placing a mixture of one part heroin and four parts daii fan, a barbituate sleeping powder, on a piece of creased tin foil, they then heated the mixture until smoke began to rise. To ensure the mix vaporized and didn't burn, they moved the heat source, then "chased the dragon" (chui lung) by inhaling it through a thin tube. This technique enabled them to feed their habit without carrying pipes, needles or other incriminating paraphernalia, and quickly spread as the Hong Kong and Chinese governments began cracking down on opiate users.
Heroin hydrochloride does not smoke well: the addition of the barbituate powder increased the amount of vaporization smoked. Later the technique was improved by replacing the barbituate powder with caffeine, creating a smokable mix called "Chinese No. 3." By the 1970s Chinese No. 3 was available in Thailand, Singapore and Malaysia: by the 1980s it had reached India and Pakistan. "Brown sugar" – Iranian heroin consisting of heroin base mixed with caffeine – became available in Europe in 1975 and by the 1980s many European heroin addicts had replaced injection with smoking. (Others added citric acid or lemon juice to their "brown sugar," thereby converting the base to a water-soluble salt and making it suitable for injection).
Heroin-smoking came later to the United States. Legal pressure on traffickers and importers meant that most American heroin was of low purity. As a result, American heroin addicts preferred injection as the most efficient delivery system. This changed with the advent of "Black Tar heroin" in the 1980s. Black tar heroin is easy to smoke, although the burning vinegar taste (a byproduct of excess acetic acid used in production) leaves much to be desired. Many young users began smoking black tar thinking they were using opium. Others considered it safer than injecting. And with the advent of fentanyl patches [Chapter 14], some users have taken to splitting open patches and then smoking the gel found therein. This is extremely dangerous, given fentanyl's potency: a sufficiently large hit may be the last thing you ever inhale.
While smoking is less hazardous than injecting, it is not without its risks. Many regular heroin smokers report impaired lung function, chronic bronchitis and emphysema. (This danger is compounded if the user also smokes tobacco, cannabis or hashish). Physical dependence can develop quickly, and many addiction specialists believe the quick rush produced by smoking causes an increased risk of psychological addiction. The intense cravings associated with crack smoking can also occur in heroin smokers. And because smoking is comparatively inefficient, many users find their tolerance rises to a point where economics force them to inject their drugs.
A number of heroin smokers have developed toxic leukoencephalopathy. The white matter of their brains begins breaking down, resulting in loss of coordination, slurred speech, and cognitive impairment. This condition has a fatality rate of approximately 25%, while many who survive suffer permanent brain damage. Between January and July 2003 17 cases of heroin-related toxic leukoencephalopathy were confirmed in British Columbia: seven of these were fatal Similar cases have been recorded in New York, Los Angeles, the Netherlands and Taiwan. The toxin which causes this has not yet been identified, and there is little information on how many heroin smokers may presently suffer from mild forms of this condition.