
What do folk mean when they say they have damaged/bad veins, or don't have any veins left? Are they ever being damaged enough to stop blood flow or do they just become too hard to inject into cos of scarring or?
All of those things can and do happen. Collapsed veins are the worst - they won't be repaired at all and lose function afaik. Certainly very reduced function at best. Sometimes the body can generate veins to bypass the area or otherwise will put more pressure on existing, smaller veins which ain't so great but better than losing limbs. You'd have to work hard to do enough damage to actually risk losing a limb but it's certainly doable - clots would be a particular risk in that area and they can be caused through damage done from injecting. When you inject repeatedly scar tissue builds up and pieces can break off blocking veins and causing all kindsa problems.
Build up of scar tissue can also make veins essentially unusable for injecting. That's what most of mine are like - especially the crooks of elbows. The veins are still there but covered with thick internal scar tissue that's excruciatingly painful to poke needles into. I'm sure a skilled phlebotamist could access those veins but not with me conscious they couldn't as even fairly gentle external pressure (pressing on the area with a finger) causes intense pain and involuntary jerk of arm to get it out the way cos it really frikkin hurts. Poking at it with sharp things is unthinkable and impossible to bear - no matter how desperate I've been I've never been able to go near those sites again.
I also have an area about an inch round now on one of my elbows where the site got infected during an extended (as in several weeks/couple months several times daily) IV stim binge. The flesh actually rotted away in an area about the size of a £2 coin and deep enough to see the bone inside. Could also see veins in amongst the mush and I was so caught up in binging that I was almost pleased cos they were so easy to access. It was the kinda stuff that makes anything Hollywood presents of drug addiction look quite tame. It's not something I'm proud of and I'm frankly amazed I got away with a relatively small scar rather than a life-threatening infection. Especially as I never sought any treatment cos that would've meant taking time out from binging.
When addiction meets needle fetishism there's real potential for doing things you could never imagine yourself - or even anybody else - doing. The most shocking behaviours become strangely justifiable to you. Why wouldn't I inject into exposed veins in a rotting pit in my elbow? They're so easy to hit. Surely a blessing in disguise, no?

Out of curiosity, why is shooting in the neck and groin so much more dangerous than other sites? I mean, if you had no qualms in injecting veins you saw in a rotten hole but still managed never to break your self-imposed 'no neck, no groin' rule, just how bad is it going for those sites?
I guess (and it is really just a guess) the groin is a tricky issue due to how deep the femoral vein is and how close it is to the artery (some big nerves are kicking about there also I believe), but just curious about the neck. Same issue with an artery perhaps?
On the subject of drugs do you think injecting is much more powerful in terms of dragging you into an addiction?
I have never injected anything, never will for the reason i believe it will push me over edge from the occasional use to addiction.
No, far from it. In my view & experience your deep into addiction before you shoot anyway
) and codeine linctus (easy to acquire OTC back then)) and using heroin fairly regularly but not really daily and was really quite lightly addicted in the grand scheme. It was more because I just wanted to know what it felt like. Wanted the "full-on" heroin experience. Had been fascinated by it since the first time I saw a friend inject heroin back when he'd just give me the occasional line to chase when he'd come visit and use my place as a hideyhole so his g/f didn't catch him at it.