The number of prisoners who tested positive for drugs has generally declined in the last five years.
Figures have been released by Home Affairs in response to a Tynwald question for written reply from Onchan MHK Zac Hall.
In 2007/08, 15 of 110 inmates randomly tested returned a positive result, which is a 13.6% failure rate.
That figure rose to 17.7% the following year, but has fallen since.
To date 108 random tests have been conducted in 2011/12, with 10 positive results, a failure rate of 9.26%.
Of those prisoners tested because they were suspected of being on drugs, target testing as it's known, nearly 55% failed in 2007/08.
That dropped significantly in 2008/09 to 25% and continued to fall for the next two years, to 20% and 12.5% respectively.
So far in 2011/12 the failure rate is 15%.
In a follow-up question for written answer, home affairs minister Juan Watterson said the increased security, passive and pro-active drugs dogs, CCTV, improved security intelligence and regular joint operations between the police and prison staff continued to act as a deterrent for people attempting to smuggle illegal drugs into the prison.