Well...mescaline wasn't synthesized until roughly 1920. It was probably in a liquid form where/when he was shot up in a hospital. Maybe there was a bit of curiosity with regards to speeding up the process by injecting it into the blood stream. I'm not sure if it would move faster that way though because I believe it processes through the pancreas initially??..but I may be wrong.I don't know
Sartre was following up study on Husserl's work..and maybe that is how Husserl did it...and he wanted to copy the method, That makes sense to me.