Scotland has more than 50,000 heroin users, it has been claimed
More than a quarter of drug addicts seeking treatment in Scotland have waited over a year to be assessed.
Hundreds have also waited more than a year for treatment after a care plan has been agreed, according to new official figures.
Rival parties said ministers had left addicts lingering on waiting lists.
The Scottish Government said excessive waiting times were unacceptable, and added that recovery was a key part of its drugs strategy.
More than two-thirds of those offered an appointment for assessment got a date within 14 days of initial referral according to the the NHS figures, which covered the three-month period between October and December last year.
But they also showed 554 patients had waited more than 52 weeks for an appointment for assessment by the end of last year.
A total of 318 patients waited between six months and a year for an assessment date.
And 210 addicts had to wait more than a year for treatment to actually begin, while a further 109 were left waiting between six months and a year.
Conservative leader Annabel Goldie said the figures were shameful, adding: "When an addict is ready to accept help, help should be given within days, not years."
Labour's public health spokesman Richard Simpson said the waiting times were deeply concerning.
"We know from bitter experience, that the success of any drugs policy depends on helping addicts to break their addiction," he said.
Ross Finnie, of the Liberal Democrats, said early intervention was vital to combat drug misuse.
He went on: "Leaving people lingering on waiting lists for a year only increases the risk of them falling out of care and back into continued drug abuse."
The public spending watchdog Audit Scotland, which says there are more than 50,000 heroin users in Scotland, has put the cost of drug misuse at £2.6bn.
A Scottish Government spokesman said the figures indicated a 10% improvement in waiting times for referral to assessment and a 20% improvement on the time waited for any intervention, compared to the previous quarter.
"Access to effective treatment and rehabilitation is critical to the goal of recovery from drug misuse and efforts must be made to improve it," said the spokesman.
"It is unacceptable that people have to wait months to get the help that they are seeking. In many instances the chance to help someone will be lost if timely support cannot be offered."