Chasing the dragon
"Chasing the dragon" (traditional Chinese: 追龍; simplified Chinese: 追龙; pinyin: zhuī lóng; Jyutping: zeoi1 lung4) is a slang phrase of Cantonese origin from Hong Kong referring to inhaling the vapor from a heated solution of morphine, heroin, oxycodone, opium, or ya ba (a pill containing caffeine and methamphetamine). The "chasing" occurs as the user gingerly keeps the liquid moving in order to keep it from overheating and burning up too quickly, on a heat conducting material such as aluminium foil. The moving smoke is chased after with a tube through which the user inhales. The process may be referred to as a "foily" in Australian English.
This method of intake significantly decreases or eliminates certain risks of heroin use, such as the transmission of HIV, hepatitis, and other diseases through needle sharing, the introduction of skin bacteria to the bloodstream due to non-sterile injection, and the stress that injection puts on veins cannot occur. Additionally, a small puff can be inhaled as a method of gauging the strength of the heroin. This may protect users from overdosing. Finally, the lungs can act to filter out adulterants that otherwise would pass directly into the bloodstream. One of the most common of these adulterants, talc, has an apparently greater potential to damage the lungs (as well as other organs, such as the kidneys) when present in the bloodstream than when inhaled.
In any case, it is always harmful to expose the lungs to any kind of smoke or heated vapor. Inhaling heroin appears to rarely lead to toxic leukoencephalopathy. There are also documented cases of both severe acute asthma and exacerbation of underlying asthma caused by heroin inhalation, potentially resulting in death. 
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