Firstly, their will be a sacrifice in bioavailability, however you really have no choice at this point. I eventually ended up in the intramuscular realm and also ended up with severe infections after a few weeks of constant injecting. By the time I was done, I basically couldn't pick anything up and sitting on my ass hurt like a mother fucker.
Heroin as a hydrochloride salt is pretty water soluble. If you happen to have Heroin Base, simply treat it with Vitamin C or what have you, which will make the drug soluble. As opposed to IM/IV in which the user wants to inject as little vehicle as possible, when going rectal, it's my personal opinion that the more vehicle the better. I often do an entire oral syringe, followed by another of just water. It is my theory that having more surface area occupied will increase absorption.
If you are constipated this is not a good route. If not, simply make up your solution in an oral syringe, actually stick the syringe into your rectum, lie flat on your belly for 15 minutes and you will feel the drug's effects. I recommend another syringe of water.
To be honest though, I often simply follow the same method standing. I just go into the office bathroom, shoot and keep it clenched. Plugging is a cornerstone of Harm Reduction. It has a cost/benefit ratio that makes it favorable to IM/IV injection. This isn't a joke: if you often have large items stuck in your ass, your rectum will be less tight and might warrant a necessary prostrate administration.
YAY HEROIN!