There's a stigma to injection-users that just isn't there for the rest of us: the "junkie" is seen as the lowest of the recreational substance users. For the average person "hey, want to shoot this coke?" doesn't have quite the same ring as "hey, want to split a line?". It's a shame, really, because injection (especially with proscription medication) is often the safest, most reliable and most efficient way to ingest a substance.
Well, for one, injecting is much more complex - there is sterile procedure to follow. It is VERY difficult in some states to acquire syringes without a proscription, let alone sterile water packets and filters. Many people are afraid of needles, or simply unaware of proper injection methods. The quality of the substances can cause major issues - tar Heroin is VERY bad for your veins, and who knows what it has been cut with or how many dirty hands touched it on it's way to you?
Lastly, the effects of injection (whether I.V or inter-muscular) are MUCH more intense, and often shorter and faster acting. It's similar to snorting coke vs smoking crack, the risk of addiction or health complications rises the more of a substance goes into you, in a short amount of time. This leads people who are more susceptible to addiction on a slippy slope down if they choose this as an ingestion route. For some, cost is a factor - the bio availability of most opiates taken inter nasally is quite low. By choosing I.V they can get the same effect with significantly less product. When you're a poor addict on the street, a $10 hit means a lot more in a needle than up your nose. Because of the desperation factor, junkies get much more public exposure than casual users and skew public perception.
For me, injection was one of my final frontiers - after doing most of what's commonly available, I started trying different routes of administration. Needless to say, regardless of the substance OR the method I choose to ingest it, I have NEVER been addicted. Opiates, stimulants, dissociatives, hallucinogens, whatever.
Some of my friends have been concerned, others curious. So far, I have yet for anyone to disown me or question my character - everyone knows I'm a stand up guy when it comes to money and safety. Some tell me to use the washroom, others want to watch or try. Most who have tried injecting have only done it the once and that was enough to know they didn't like it. A couple went on to use sporadically for a couple weeks or months, then decide it wasn't a good idea.
I am not an addict. No one I have come into contact with has become an addict. As with anything else - be safe, have fun, know your limits, and NEVER be afraid to ask for advice or help. Ignorance kills!