Place an IV catheter in your vein, just like they do when you go into the ER. Google it if you don't know what I'm talking about.
Think about it: if you've ever been to the hospital and they needed to give you IV medication, how many times did they ever poke a vein? ONCE. That's it. The thing is made so that a soft tube to carry fluid in and out of the vein is connected to a plastic piece, and that whole thing slides over the needle and attaches to the part behind it. On the end of the catheter, there's an injection port. To ensure that the vein stays open, you can attach a stub of IV tubing to the catheter after you place it, then you flush it with saline (medicaleshop.com sells saline flush syringes really cheap) and attach the cap. This whole setup is called a "heparin lock". There are heparin lock kits you can buy on ebay. They're kind of expensive, but they will give you the list of the stuff you need and you can also get it cheap on like aliexpress. It takes some effort to get the hang of it, especially because all the information out there on how to do it is geared toward people PUTTING CATHETERS IN OTHER PEOPLE, not themselves hahaha. But f that. One day, I thought to myself, "Wait, why the f am I poking myself over and over? I should be using a goddamn IV catheter!" First time I tried to do it, I messed up and blood was flowing out all over everything, lol, but I can do it fine now. So like do it by the sink.
If you REALLY wanna do it right, hang a bag of normal saline (you can buy veterinary 0.9% sodium chloride + sterile water bags without a script online pretty cheap) and just have it flowing into the vein. Then you use the injection port of the catheter when you want to give yourself a shot. You can get setups where you don't even need to use a needle to access it at all. They use connectors called "claves" instead. That can be nice because no needle stick injuries possible when u get all spun out.
Now I've gotten to the point where I have my own portable infusion pump to use if I want lol (CADD Solis). You can buy these cassettes that clip onto the bottom of it, then u put all shit in there. You just dissolve it in some saline/water you have in a bowl in front of you or whatever. Let's say u got a gram of coke. You can use a 100ml reservoir, so that's 10mg/ml, which dissolves easily. So you've got everything dissolved, then you throw a filter on a 100ml syringe and draw it all up into the syringe through the filter. You attach the reservoir to it and empty the syringe through it into the reservoir. Then, you use a tube called an "extension set" to hook up the cassette to the catheter. The company makes backpacks and other kinds of bags that you keep it all in, and it can run a LONG time on AA batteries. Then u program it so that u just have to press a button and it gives you a dose.
You don't need the pump tho. You can leave that out of it and just put all your drugs into solution and filter it back into the bag. Then you draw it out of the bag every time u wanna take a shot. Those heparin caps I was telling you about are made out of a material that you can penetrate with a needle. I didn't plan on getting all into this, but whatever. Lately I've been thinking to myself that it really is insane that it isn't normal for people to use catheters. There is no reason to destroy your veins, or even poke a vein with a needle more than once in a day.