WATER WOES: National's Jacqui Dean is the latest MP to fall for a hoax, calling for a ban on "dihydrogen monoxide" - or water.

National MP Jacqui Dean has been caught out by a long-running hoax that seeks to trick gullible MPs into calling for a ban on "dihydrogen monoxide" - or water.

Ms Dean, the MP for Otago, a leading proponent of a ban on party pills, appears to have been duped by a letter from a constituent asking her to raise the issue of dihydrogen monoxide (DHMO).

A letter, signed by Ms Dean and sent to Associate Health Minister Jim Anderton, the minister in charge of drug policy, last month, asked if the Expert Advisory Committee on Drugs had a view on banning the "drug".

Mr Anderton yesterday took the opportunity to rub Ms Dean's nose in the embarrassing blunder.

He said dihydrogen monoxide "may have been described to her as colourless, odourless, tasteless and causing the death of uncounted thousands of people every year, and withdrawal from which, for those who become dependent on it, means certain death.

"I had to respond that the experts had no intention of doing so."

It is not the first time MPs have had a brush with the hoax substance.

In 2001 a staffer in Green MP Sue Kedgley's office responded to a request for support for a campaign to ban dihydrogen monoxide by saying she would be "absolutely supportive of the campaign to ban this toxic substance" if she had enough time, which she did not.

National used the letter to try and embarrass the Greens - who were at the time waging a campaign against genetic engineering - as scientifically illiterate.

Wikipedia says the hoax has been running since 1989 with the aim of illustrating how a lack of scientific knowledge and an exaggerated analysis can lead to misplaced fears.

It was initiated by three University of California Santa Cruz students and gained widespread public attention in 1997 when a 14-year-old student, Nathan Zohner, gathered petitions to ban DHMO as the basis of a science project, titled "How Gullible Are We?".