We typically recommend at least 5/8" rigs for IM, usually we stock 27G5/8 and 23G1 1/2" (1.5")- those we recommend as the best IM rig we can stock - to ensure it his the muscle. Using a typical 28G1/2 we don't endorse, you'll likely end up with an abscess prone SC injection, which will get you high. Just like a miss will. This is all just at the syringe access program I support and volunteer at (and occasionally am a "client")
Sounds like you got longer rigs (>1/2" tip), that's good, very good. Many people, myself included, used to use our "regular" 28G1/2 to IM. If it took me too many tries and too many new points to hit a vein. I usually used my butt, but again I've learned from my HR work that the upper arm, where you typically get inoculated as a kid (or adult if you travel overseas, flu shots, etc...) is the best spot due to good blood flow.
Clean everything, and prepare shot as if it were for IV, insert fully at spot on upper tricep, try to be at a perfect 90 degree angle, pull back to be sure you're not in a vein if you are then you can decide on the fly if you want to take the opportunity or pull out or backload a new rig, add a little clean, cold water if there's much blood in the solution, shake to mix, and go around again - and plunge slowly. I've never got a vein doing IMs, more likely you'd hit an artery with a 1.5" needle. If you do pull out immediately! I never got an artery with about 50 IM injections and our clients don't report that they have done so. Still, it can happen.
You won't get a rush, I rarely did IV much less IM unless I did too much by IV. Your danger is in using too much b/c it will hit delayed, but you'll get the full effects of the loaded rig. Don't redose for at least a half hour or more, you risk OD, it's happened to me with misses, a few times I immediate reloaded and hit properly after just a few minutes. Too much, too fast.
You won't loose any dope if it's done properly, but your high may feel a bit muted. I never minded to much, and sometimes IM'd straight away if I was sick or in a big hurry (using a family member's bathroom, public place, etc...)
Best wishes for your recovery, you are going for a complete switch and end to dope use, correct? I hope so, there's so much more to life, don't let it pass by.
Be clean, deep abscesses suck, and your risk I believe is higher by IM'ing versus a good clean IV hit, so if possible I used wheel filters, but rarely was that practical.
Best. Wishes. Get well. Love life!
