KEY POINTS:
Professor John Henry, who has died aged 68, was one of the world's leading authorities on drugs and poisons.
When, in September 2004, the Ukrainian opposition leader Viktor Yushchenko, fell seriously - and mysteriously - ill during the election campaign, Henry was sent a photograph of the patient.
He immediately concluded that, because of the characteristic pattern of acne, Yushchenko had been poisoned by dioxin.
Henry was also called in as an adviser in the more recent cases of the former KGB agent Alexander Litvinenko and Pakistan's cricket coach Bob Woolmer.
Henry believed that Woolmer could have been poisoned with aconite, which works like cyanide.
As a 20-year-old medical student, Henry joined Opus Dei as a "numerary member", committing himself to a life of celibacy.
In 1982 he was appointed consultant physician at the National Poisons Unit at Guy's Hospital.
Henry insisted that cannabis was much more dangerous than simple tobacco; it eroded users' volition, drive and dignity, destroyed the personality and had devastating effects on society.
He was also one of the first to warn that the risks of taking Ecstasy had been underestimated; such were his vivid descriptions of the results of nightclub abuse that at one stage he was known colloquially as "Mr E".
