What hallucinogens can be absorbed through the skin?

Answer 1

LSD (acid) has already been metabolized by the time it exits your body through sweat. You would have to ingest a ridiculous amount of LSD for it to even be present in sweat. A few drops of acid as metabolized into hundreds of drops of sweat comes out to a negligible amount leftover, if any.

Answer 2

There are certain plants which contain alkaloids which can be absorbed through the skin, such as for instance the Datura plant contains scoplamine which can absorb through the skin. As for something like LSD or Psilocybin no they cannot go through the skin.

Answer 3

Quite simply, yes. However, the bioavailability is very low, meaning that it is terribly inefficient way to deliver LSD to the brain.