The Safer Bristol partnership found three quarters of inmates had a good or fair experience of having their drug problem dealt with.
But 19% reported a poor experience of the drug treatment service.
For the first time, HMP Bristol agreed to trust prisoners to carry out the review with the support of Bristol's User Feedback Organisation forum.
"Working with those who actually use our service and to be able to give them a platform to influence change is very important for the future development of our service," said Tony Meecham, head of drug services at the prison.
The review was first carried out in 2009 and then again in 2011.
It found waiting times for treatment had significantly improved, with most people being seen on the same day as arrival.
"The responses show that treatment is more prompt and therefore more effective than three years ago," said Paul Moores from Safer Bristol.
"This is helping prisoners become confident that they can become drug free in prison and afterwards.
"Receiving treatment as soon as they enter prison is vital as withdrawing from drugs without treatment can cause huge physical and emotional problems."
Despite the improvements, 19% of inmates reported a poor experience of the drug treatment service.
Areas highlighted for improvement included the need to support those with mental health problems, and a specific treatment for people using crack cocaine.
"It [drugs] ruin your life," said prisoner Paul Bryant, who has been at Bristol Prison for seven months and has received support for his addiction to heroin.
"You lose your family, your support, your job, places to live and you lose your liberty. I've worked through the medication and I'm now completely clean.
"I'm 'number one' on the wing, so I'm in charge of all the other inmates to make sure they do their jobs.
"It [the prison] supported me through it and then I worked my way up from serving food to being number one on the wing."
HMP Bristol receives male prisoners and a limited number of young offenders, both convicted and remand, from all local courts.
It also acts as a category B facility for the West of England.
Inmates at Bristol Prison in Horfield are getting better help to come off drugs, according to a new report.