The best places to IM heroin are your upper-outer arms (also called upper-lateral). To find this area on both arms, place two fingers on your arm starting at the top of the shoulder. The deltoid muscle covers your shoulder and upper arm; the area to insert the needle is two finger-lengths below the top of the shoulder. To ensure that you are in the muscle, you should be using a 3cc, 1 inch, 25g needle (if you are a hefty person or a bit on the very "bigger" side, you might need to use the larger 3cc, 1.5 inch, 22/23g needles instead).
If using a 1 inch needle on the deltoid muscle of the upper arm, insert it all the way in; if you push in and hit a hard surface (your bone), simply pull back slightly and then administer the injection. Basically, if the needle hits up against the hard bone surface, the muscle is right next to it and pulling back just slightly will ensure you're in the muscle. This technique is actually taught to nurses when giving IM injections such as certain vaccines- If you hit bone, pull back slightly, then administer the injection.
Also, as a note, it is NOT dangerous to accidentally poke the surface of a bone while attempting IMing on the upper arm. SWIM's local needle exchange nurse explained that during her own training, she was instructed to push the needle in far enough to feel it hit the bone of her patient; after this occurred, she was instructed to pull back slightly and inject- because "that way you know you're in the muscle."
Two other primary areas are the upper-lateral thighs (meaning upper-outer thigh). This is the meatiest part of your thigh towards the top and on the side, below your hip. You should be able to pinch the area to feel for the meatiest part, and administer the injection with a 1 inch, 3cc, 25g needle. Unlike the arm, you probably won't strike bone by accident, so to ensure that you have entered the muscle simply push all the way in with the 1 inch needle.
The third two main areas for IMing are harder to explain; it is the area just above each butt cheek, then moving outwards towards the hip. First, place a hand on one of the cheeks of your ass. Then move your hand up to where you hit the small of your back/to the side of the base of your spine. Move your hand slighly outward, stopping just before you've gotten to the side on your hip (so that your hand is still technically *behind* you, and *not* on your side/hip. If a friend was standing in front of you while you were trying to find this area, they would NOT be able to see your hand because it is still behind you and NOT on your hip). You should be able to pinch and feel for a fairly meaty area which is where you would perform the injection(using this procedure on both sides of your ass). You could call this area "Upper butt, slightly outward." For a normal sized person, use a 1 inch, 3cc, 25g needle, sticking ALL the way in to make sure you pierce the muscle there.
***Injection Technique***
Many people here have stated that IMing any street heroin is asking for an abscess. SWIM has also seen a few people here group "IM/SC" together as if they are the same thing, or very similar. They ARE NOT! IMing refers to injecting DEEP and into a *muscle*. SC refers to injecting into tissue below the skin, and is also what happens when someone misses their shot of heroin and gets a white blister as a result.
"Asking for an abscess" was a phrase SWIM frequently used to use regarding IMing before becoming correctly educated about IM heroin injection from needle exchange nurses. In reality, SC injection is "asking for an abscess"- Although this *can* vary from area to area. In higher-purity areas, SC injection is far less likely to lead to an abscess because purer heroin absorbs much quicker than poor quality heroin. Usually, "SC injection" is done by accident when somebody misses their shot under their skin.
A nurses' words regarding the safety of IM injection of heroin:
Nurse: "Most people who have abscesses in our area [black tar/poor quality] from IMing heroin just didn't do it correctly. You need to use the correct size syringe, usually a 1 inch needle, 3cc. People here will attempt to IM with a 5/8ths inch needle and a thick black solution with it since those are only 1cc syringes- then they usually fall just short of the muscle, or their solution was so thick that it couldn't absorb, or they were just plain unsterile doing it."
SWIM: "But what about the heroin in this area? It's very poor quality- I've read that abcesses form inside the muscle- it sounds terrible, I'm scared to ever try it even though I have no veins left!"
Nurse: "Did you get any IM syringes here? We're still open for exchange. You see, there's a good amount of bloodflow inside the muscle, and as long as you actually get into the muscle, there should be no problem assuming you use sterile equipment and alcohol. Also you must make sure to water down the solution the best you can- feel free to draw up the entire 3cc syringe with extra water-all the way to the top- to make it as diluted as possible. Remember to use site rotation as well!"
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Make sure to use all sterile equipment, filter with cotton, NEVER reuse your IM syringe, and swab down the target muscle thoroughly with alcohol! SWIM prefers to swab with alcohol before and after the injection, and prefers to inject slowly with the entire dose of heroin diluted in 3cc of water for ease of absorption into the muscle.
Initially, after injection, you will notice slight redness at the site and a little soreness- similar to having a tetanus shot at the doctor's office! If performed correctly, the redness will subside within an hour, while some soreness might persisit. SWIM uses a trick taught to her by a nurse, which is to rub the injection site with an alcohol swab in gentle circular motions after injecting. This encourages the solution to absorb faster (also, baths and hot compresses will also work) and will help alleviate the minor soreness faster.
Nurse: "IMing has a slower onset, but it will definitely get you well, and since there's such a good amount of blood in the muscles, you may even get a.. well.. high."
SWIM nodded out during the making of this post response, so SWIM will vouch that IM injection does produce decent effect. SWIM noted that onset of the effect was five minutes or less after finishing the injection, and would last a normal 4-6 hours as with IV administration.