The conservatives have launched an attack on the government as figures show the number of mental health hospital admissions due to cannabis have risen 85% under Labour.
Statistics obtained by shadow health secretary Andrew Lansley also reveal that there has been a 63% increase over the last five years.
Health minister Rosie Winterton said in a written commons answer that in 1996-97, there were 510 admissions, rising to 946 in 2005-06.
But it should be noted that admissions are not the same as the number of patients, as one patient may be admitted more than once in a year.
Mr Lansley said: "Awareness of the link between mental illness and cannabis has increased over recent years, as has the strength of the drug.
"Both these factors have contributed to the sharp increase of hospital admissions on mental health grounds.
"That's why the Conservatives have opposed the downgrading of cannabis and pledged to have it reclassified."
Cannabis is a class C drug at present, but there are calls to move it back to class B.
A Department of Health spokesman said: "Our message is clear - cannabis is harmful, is an illegal drug and should not be taken."
Marjorie Wallace, chief executive of the mental health charity SANE, said: "SANE has been saying for years that there is a link between psychosis and cannabis, particularly in its more potent forms, and so this increase in hospital admissions comes as no surprise."