One in three prisoners is taking prescription drugs, costing taxpayers nearly $1.3 million a year.
The cost of prescriptions for prisoners rose from $344,903 in 2000 to $1,279,379 in 2005-2006, figures supplied to National MP Jackie Blue show.
The percentage of male and female inmates on prescription drugs rose from18 per cent to 32.
Dr Blue obtained the information through parliamentary questions, but said getting information on the drugs used was impossible.
"Efforts to get information about the type of pharmaceuticals used and the pattern of use over these years has hit a brick wall."
The Corrections Association is fighting moves to make prison guards responsible for providing inmates with their medication when nurses are not available.
Corrections Association president Beven Hanlon told the Dominion Post that the group was arguing its case in the Employment Court.
"If you hand out medication and the guy tops himself on it or gives it to someone who tops himself, who is in trouble?
"It will be one of us who loses our job, not the manager who said to do it that way."
Mr Hanlon also had concerns about guards' training to do the job.
"Nurses are trained to do these things. What happens if we give the wrong medication? To me a pink pill is a pink pill, but a nurse can tell the difference."
Corrections spokeswoman Bridget White said prescriptions were more expensive and the number of prisoners had increased.
Some inmates arrived at prison needing a range of treatment.
