Since 1995, the percentage of primary heroin treatment admissions ages 29 and younger who reported injection as their usual route of administration has increased steadily, from 53% to 72% in 2004.
At the same time, the percentage of older heroin treatment admissions (age 30 and older) who reported injection as their usual route of administration has decreased from 82% in 1992 to 59% in 2004.
One possible explanation for this trend is that younger heroin users perceive less social stigma and/or risk of contracting HIV/AIDS associated with injecting drug use than do their older counterparts.
I find this stigma theory doubtful. I’d guess it has more to do with fluctuations in purity. Prior to 1995, smoking or snorting heroin wasn’t much of an option due to low purity–injecting was the only efficient method. As the cut improved in the mid to late 1990’s the threshold for heroin initiation dropped when new users could smoke or snort it without violating injection taboos. Those who develop more serious addictions are likely to progress to injection. As purity increased, existing injection user who were less severely addicted or were less price sensitive could switch to lower risk snorting or smoking.