Hey
@Oak cliff_23 
First, to answer your question. Please read through to the end though.
All forms of injection, that is the kind of injection that a person can do themselves with just a needle (I'm obviously gonna exclude stuff like epidural, intrathecal etc.) result in ~100% absorption. These routes of administration include:
Intravenous - Into a peripheral vein
Intramuscular - Into a muscle
Subcutaneous - Under the skin, between the dermis and muscle
The bioavailability for each is ~100%. The primary difference between them and how they are used in medicine is the diffusion rate of the medication. In the order listed, this is quickest to slowest in terms of how quickly the drug will be taken up into the bloodstream. An intravenous injection is essentially instantaneous. Intramuscular injection will typically be felt within 5 minutes, though it's typically not fully absorbed for 10-15 minutes. This is still faster or at most, the same rate at which insufflation works. Subcutaneous injections are usually given specifically for their slow rate of diffusion. A medication commonly given by the SC route is insulin. There is much more variability with SC injection. Just know that it's a slow process.
Now we really need to talk about hygiene and the quality of your drugs. I'm definitely not criticizing your technique/hygiene, though the fact here is that your best attempts have led to the loss of your easily-accessible peripheral veins. With this in mind, I feel it's safe to say that switching to intramuscular injection is going to cause more problems than solutions. If you're using street Fentanyl/Xylazine like we have in North America, I'm gonna say definitely do not start using it intramuscularly; it's an absolute mistake.
The equivalent to a vein collapsing is a muscle becoming hard. When I say hard, I mean they get seriously hard, reportedly not even allowing a needle to penetrate. Even before the current, tainted drug supply, it never seemed that people were able to use via IM for very long.
All conditions being equal, IM is just a more hazardous ROA than IV. When you inject something into the blood stream, it's taken and spread around the body immediately. When you inject something into the muscle, it is going to "sit there" for a significant period as it is taken bit by bit into the bloodstream. This is what tends to cause complications at a higher rate. Basically, you have a limited amount of time before you start getting problems. Abscesses are significantly more common.
If I were you, I would decide now that you're gonna have to find a different route of administration. I'm not sure why you say you can't vaporize your Heroin. If you could explain that would help us help you. It's the only ROA that is directly comparable to injecting. Alternatively, I would suggest experimenting with rectal administration. It's not as good as an injection, but you'll get ~70% BA with an onset rivalling that of your IM injections. Rectal administration is safe, easy and sustainable, which is what you need. You can either make the change now or wait until 3 months from now when your muscles are hard, you're body is bruised and you're dealing with infections. You'll be in the same spot you are right now, just with more consequences. I highly suggest taking the plunge now. We will help you however we can.
If in the end you decide to keep doing the IM injections, do your utmost to keep the situation clean. Fresh needles, sterile conditions and the rotation of sites.
