Henry then returned to his career in medicine as a registrar at Guy's Hospital, where he showed great compassion for his patients. In 1982, he was appointed consultant to the National Poisons Unit at Guy's, where he was successful in saving many lives, especially those of children who had ingested poisonous household products. He carried out research in toxicology to discover how those poisons worked and how to counteract them.
Henry was able to explain medical matters in layman terms, and was well versed in all aspects of drugs and poisons.
He took a special interest in the damage done to young people's lives by illegal drugs. He insisted that cannabis was much more devastating than simple tobacco, taking away the user's free-will and dignity, and destroying personalities and damaging society.
He also explained how ecstasy (MDMA) and amphetamines could cause death by hyperpyrexia and dehydration. He was among the first to claim that the dangers of ecstasy were underestimated, and he briefly had the nickname "Mr E". He was called as an expert witness for the inquest into the death of Leah Betts, who died after drinking too much water after taking an ecstasy tablet at her 18th birthday party, and whose case became a cause célèbre.
