Someone who engages in the use of highly
physically addictive drugs without becoming an
addict. These people are very rare, and their ability to do so depends partially on personality and strength of will and also on physical makeup - some people simply form less of a
craving, and some form none at all.
The term comes from junky slang, although it has expanded from simply heroin use. Recreational smokers get called this as well as puffer, although many are probably on their way to addiction.