For UK users and anyone else needing to use acidifiers in disolving their herion.
There seems to be some debate about which acid is safer. In actual fact there is no evidence that one acid is safer than the other. If you think about it, it's the pH of the solution that determines how damaging it is, and this is determined purely by how much excess acid is added. If precisely the correct amount of acid is used, the hydrogen ions will be 'mopped up' by the heroin.
There is however an argument for choosing vitamin C over citric. More vitamin C is required to provide the same amount of acidity, allowing a greater margin of error. i.e. you are likely to add a greater excess of acid when using citric than when using vitamin C.
There is also an argument against using vitamin C. Too much of this acid can lead to the formation of kidney stones. It would probably require quite a bit more than anyone is likely to inject, but in theory it's still a risk.
Personally Swie believes the greater margin of error afforded by vitamin C outweighs the very small risk of kidney stones, and she uses VitC.