I use up to 10ml water when plugging. Because I have 10ml syringes. Some drugs which are caustic or irritating, like Ethylphenidate and 4-FA, become much less caustic or irritating in a dilute solution, and it works just as well as with a small amount of water. It's surprising how quickly the rectum can absorb a solution of drugs and water when it's put to the task!
On the other hand it makes no difference with AH7921. I've fallen out of love with that drug, because it seems pretty dodgy in this regard, especially now I have a fairly quick poppy pod tea system.
I always use filtered water, because it's going into my blood stream, but on the other hand moist shit passes through the rectum on a regular basis and that's not a problem, so presumably the lining of the rectum is quite good at filtering out unwanted stuff. Still, better safe than sorry.
Because plugging has brought me closer, physically and emotionally, to my rectum, it is much cleaner than in my pre-plugging days

There are no skidmarks in this house.
edit: Just been to Asda to pick up a few bottles of wine. I was shocked. No queues! What's going on? Austerity bites.