Professor David Nutt, who is chairman of Britain's Advisory Council on the Misuse of Drugs (ACMD), made the comment in an article published in the latest Journal Of Psychopharmacology. In his article, entitled "Equasy - An overlooked addiction with implications for the current debate on drug", Professor Nutt wrote: "The point was to get people to understand that drug harm can be equal to harms in other parts of life."
He used the term Equasy to refer to equine addiction syndrome.
He continued: "There is not much difference between horse-riding and ecstasy.
"This attitude raises the critical question of why society tolerates - indeed encourages - certain forms of potentially harmful behaviour but not others such as drug use."
The piece was written in his capacity as an academic and not as chairman of the ACMD, which is an independent expert body that advises the British government on drug-related issues.
A spokesperson for the ACMD said: "The recent article by Professor David Nutt was done in respect of his academic work and not as chair of the ACMD."
The professor pointed out that equasy resulted in more than 100 deaths a year.
Ecstasy is responsible for about 30 deaths each year in Britain, but the number has been rising steadily each year. Deaths normally result from organ failure.
Professor Nutt's comments came less than two weeks after the British Government announced plans to reclassify cannabis as a Class-B drug instead of its current Class-C status.
The move came after the ACMD concluded the drug to have serious consequence for mental health.
Stronger cannabis, often referred to as skunk, now dominates the British market. It reportedly makes up 80% of sales, compared to just a third of sales seven years ago.
Reports in Britain suggest that ecstasy may be reclassified from A to B.
Meanwhile, Britain's Home Secretary Jacqui Smith voiced her concerns over Professor Nutt's views during question time in the House of Commons.
"I made clear to Professor Nutt that I felt his comments went beyond the scientific advice that I expect of him as the chair of the Advisory Council on the Misuse of Drugs (ACMD)."
She said the professor had apologised to her.
She continued: "I'm sure most people would simply not accept the link that he makes up in his article between horse riding and illegal drug taking.
"For me that makes light of a serious problem, trivialises the dangers of drugs, shows insensitivity to the families of victims of ecstasy and sends the wrong message to young people about the dangers of drugs."