I remember reading that it won't give you stable levels and is dangerous to take it IM not that it is rendered useless.
Well, the fact that it is rendered useless is dangerous for diabetics, so that is where it comes from.
1) It can cause build up of small fat nodules when injected in muscle. This is rare when subq.
2) It's absorbtion rate is very irratic when in muscle. It can be very very fast, or very fast, and is generally much faster than subQ.
3) It can be broken down to a useless form depending on the presence of certain metabolites in the muscle tissue (probably the cause of 2 above).
4) It can be painful for hours after injecting if you hit muscle.
5) The rate of absorbtion can be effected if you have recently exercised the muscle in question.
For someone not diabetic, the above may not be "dangerous" because your life/health is not depending on being exact, but IM is not reliable and a waste. If the only difference is how far you stick it in, why not do subQ and do it right???
I can vouch for the above considering I poke myself about 7 times a day. Being lean, I have hit muscle a couple times, and with novolog (an analog- fast acting) wow did it come on strong and a couple times it was sore for a while - and stunk cause it was right on my belt line so it kept rubbing all day.
The key to insulin use is to match your insulin dose to your carb intake, so that you 1) can force as much of your carbs into your muscle (along with anabolic repair), and 2) at the same time not have to keep eating and eating because you are going hypoglycemic because you used too much insulin or your insulin release is irratic. Ideally, you use a fast acting insulin, and figure out through testing how many carbs you can clear based on a certain insulin dose.
Here's a scenario. One day you inject in your ab muscle 2 units, and eat 50 carbs within 15-20 minutes. After about 2 hours you are at a stable 110 blood glucose level. You did Biceps that day, so you worked your core very little. The next day you hit the 2 units in your ab muscle, but this day you did a lot of compound movements that worked the core area more than the previous day. You eat your 50 carbs again, but suddenly at 30 minutes in you are feeling shaky and lightheaded and kinda out of it. Oops, your blood sugar is low and you feel like crap. You feel like you are losing your mind. You're a little disoriented. So you panic, and start eating sugar and more sugar, more than you planned. You end up pushing your carb intake past the anabolic window and the excess insulin starts pushing fats around where you don't want them to be. But the low glucose level is a horrible feeling to begin with, alone.
If you inject subQ in the abdomen, you most likely will always have the same absorbtion rates and be able to match your intake amounts to best utilize the hormone, and most like would not be faced with the above.