I have to be careful what I say. The first thing to understand about insulin injection, it is some like but mostly NOT like injecting drugs (of any type). For a start it is a hormone, an altered hormone, but after all is said and done it acts like a hormone. Like drugs, the faster the route the quicker the half life (ill get backnto this). So for insulin, there are three main routes, subcutaneous (SC), intramusular (IM), and intravenous (IV). Each one increases with time taken to begin reducing BG. SC being the slowest (all things being even) and IV the fastest. Here is where it takes a turn though, with SC insulin the time to wear off can be (for fast acting) anywhere from 1 hour and 15 mins to 5 or 6 hours. For IM the half life shortens to maybe half that (in me it is an hour and a half), so it will begin quicker but wear off much faster. IV generally being the fastest (although often it can be the same as IM) also is removed from the body very very quickly, sometimes in minutes. When in hospital IV insulin is usually given in a drip because of this fact, it wears off often before it has a chance to act, so is given in small amounts very often, usually in cases of DKA. Another interesting quality of insulin is that it must go through a metabolic process, it wont just start working fast because you introduced it quicker. Many think erroneously that because they have IV insulin it will work immediately and they will come crashing down. It can and generally does start working within 10 minutes but may take up to an hour or two to complete its full cycle, however it can and often does wear off and does very little. You can see there is more to changing routes than meets the eye and this intro, is by no means rigorous. IV is never recommended or needed in the home as it can lead to a host of problems and more importantly it provides very little real time advantage over IM.
Regards IM, I thought it would hurt also but it doesnt really, although like SC it can hurt sometimes, really bad. Ive had some excruciating SC injection over the years, one every few years or so that’s so painful I pull it out and jump up and down and must have another go but none this painful with IM, ymmv. The biggest problem I have had with IM Is hitting a blood vessel and blood coming out of and running down my arm. There is a video of a girl on youtube showing how easy it is. Dont worry, your more likely to not go deep enough than to go too deep