It means that someone is in an extremely altered state of mind after taking drugs.
People may take so called "recreational" drugs to "get high" meaning some kind of pleasant feeling or sensation.
"As hell" is used to emphasise something, usually with a negative connotation. For example someone with a hangover might say "I feel as rough as hell", meaning that they feel extremely unwell. Although it can be used in a positive context as well, for example "He is as smart as hell" meaning "He's extremely smart".