This is tricky territory man. First off, good for you for coming to the conclusion that injecting them into a vein is a bad idea. Believe it or not, we've engaged in many debates with others on the topic and it's sometimes difficult getting them to come around.
Yes, my first choice would be to administer the substance rectally considering the benefits to bioavailability and such. Given that we're talking about months and not days/weeks, I definitely had an audible "gulp" when I first started reading this thread. Yes, it's very possible that something could be in the blood that could be quite harmful if it has survived this long. Given that there is a nice supply of blood for whatever organism to feed on, my limited knowledge of biology suggests that something could definitely survive for a few months in that environment.
Firstly, I don't believe it can be used without doing something serious to attempt to sterilize the solution. My first choice as a junkie would be my old friend Chlorine Bleach. It's great at killing stuff. It can also leave bathrooms smelling hygienic and fresh, especially with a little pine sol, however, that's for another time and another place.
In the best set of circumstances:
There will be no extremely dangerous organisms left in the syringe. The worst complication that will be present will be a serious blood clot. This is normal, as this is just what blood does when it's no longer moving. This would be incredibly dangerous if it were to be injected, with a likely outcome being a severe pulmonary embolism. Luckily, we are not even remotely considering that.
In the worst set of circumstances:
There is some kind of dangerous organism/bacteria that has grown inside the shot over the past several months which will cause you to become sick, potentially badly. There is unfortunately no textbook for dealing with this kind of situation. With this in mind, I'm going to throw at you my completely, shot from the hip idea. I highly suggest not doing this, and I'm only writing this from the point of view that you will likely end up doing something fairly dangerous anyway, if I can maybe, in some way reduce the potential harm, I hope I'm able to.
Empy all contents from all syringes into a single container, preferably a metal "cooker" or the bottom of a can; whatever you can fine. Heat the solution until you are left with something relatively homogenous, without lumps, bumps or other weird shit. Do not overheat or you will risk destroying your drugs, heat just enough to get them warm enough to begin congealing.
Next, take your chlorine. On the side of the bottle, there will be instructions for making a properly ratioed solution. Make that solution. You don't need a whole gallon of cleaning solution, so fee free to cut it into whatever fraction you feel like.
Next you are going to apply a reasonable amound of chlorine solution to your original solution and mix it all up. I would advise continuing to agitate/mix this solution over the course of at least 10 but preferably 15 minutes. At the very end, give it a little more heat. Just when it starts to boil, cut the heat. Let the solution sit for another several minutes, probably about 15 but you be the judge.
Once it is cool enough, you will then rectally administer the entirety of the solution. Yes, injecting pure bleach would be extremely unhealthy. We are going to be using an extremely small amount of bleach , so please don't let it freak you out. If you have questions, lmk ASAP