There is no evidence to suggest that the millions spent on trying to change prisoners' behaviour is working, a public spending watchdog has said.
Access to prisoner behaviour programmes is variable
In its report on Friday, Audit Scotland also said there were long waiting lists for some rehabilitation programmes.
When inmates are released from prison there is a 50% chance they will be back within two years.
Last year the Scottish Prison Service (SPS) spent £30m on training aimed at preventing reoffending.
In its report, Audit Scotland said that provision was patchy despite the money.
When they enter prison, 85% of offenders are thought to have a drug or alcohol addiction.
Less than half of users receive treatment.
Deputy Tory leader Annabel Goldie said the report contained some "particularly shocking revelations, in particular the lack of a comprehensive drugs strategy".
She said: "The public will rightly wonder why our prisons cannot be drug free. Prison offers the one place where the authorities should be in control - it should offer the chance for prisoners to undergo drug rehabilitation and to live in a drug-free environment."
However, the SPS said that every prisoner has access to basic addiction support and there are plans to evaluate their programmes more effectively.
Scottish National Party MSP Stewart Stevenson called on the Scottish Executive to ensure the prison service developed a plan to monitor and improve correctional opportunities.
"Scotland has an appalling reoffending rate and it's about time the Scottish Executive started to take some advice about ways of combating the problem," he said.
Deputy Auditor General Caroline Gardner said: "The provision of education, vocational training and behaviour management programmes has increased, but more information is needed about their effectiveness and value.
"The Scottish Prison Service recognises it must measure the effectiveness of its correctional programmes.
"Monitoring results is essential for evaluating how well public money is being spent.
"We need to know what difference these training, education and life skills programmes are making to the way people live their lives after leaving prison, and in particular how they affect reoffending rates."
The watchdog found that prisoners' access to skills courses and behavioural programmes was variable.
Access often depended on available resources at each prison and the duration of a prisoner's jail term.
New facilities are being developed at several prisons, but the rising prison population means the pressure on "correctional opportunity" has increased.
The report also found:
- Scotland's prisons deal with more than 30,000 prisoners every year
- The average prison population rose from 5,400 in 2002/03 to 6,620 in 2003/04
- The SPS spent £30m on correctional opportunities in 2003/04, some 12% of the full cost of operating Scotland's prisons
- The SPS performance results show 2,100 prisoners completed offending behaviour programmes and approved activities
- And 430,000 hours of education were received by prisoners across all 16 Scottish prisons in 2003-04.