Nineteen members of the Defence Forces have been discharged after failing random drug tests, with the cases of a further three yet to be finalised.
One of those who tested positive is an Army private who claimed his pizza was spiked with cannabis resin as a prank by an acquaintance.
A member of the Air Corps who tested positive claimed he passively inhaled cannabis as it was being smoked by two friends as they all travelled in the same car.
While the numbers testing positive remain relatively low, new figures show that more soldiers, airmen and seamen are testing positive now than when the random testing regime began five years ago.
Figures released by Minister for Defence Willie O'Dea reveal that seven members tested positive both last year and in 2005.
This compares with four positive tests in 2004, and four in 2003, the first full year of testing.
The regime was introduced in November 2002 but only a very small number of tests were carried out in that year, none of which were returned positive.
In a statement to The Irish Times last night, Mr O'Dea said he was satisfied with the level of testing within the Defence Forces.
"The use of drugs by personnel will not be tolerated," he said.
"The fact that so few members of the Defence Forces test positive is evidence that the system is working, and I am confident that the educational aspect of the scheme will ensure the numbers will remain low."
He said the Defence Forces had embraced the drug-testing culture since random testing was introduced.
A spokesman for the Defence Forces said the testing process acted as a deterrent.
He added that the system was working well and would be continued.
Between 1,100 and 1,200 of the Defence Forces' 10,500 full strength are tested every year. Members are required to provide a urine sample on demand, to be tested for illicit drugs.
Any member who refuses to provide the sample or whose test returns a positive result faces dismissal.
The seven members who tested positive in 2006 included one reservist sergeant, five privates and a recruit.
Five of them have been discharged, with proceedings against the other two yet to be concluded.