North Wales Police and Crime Commissioner Arfon Jones, a former police inspector, says the radical idea could also prevent overdose deaths in prisons.
Mr Jones said if justice authorities were serious about reducing harms and violence in prisons, they should be addressing the cause such as the cheap synthetic cannabinoid spice that is rife and can be deadly, as opposed to cannabis.
He said that use of illegal drugs was widespread in prisons and many prisoners lawfully receive heroin substitutes such as methadone and buprenorphine to manage their dependence.
Others that are commonly prescribed include strong analgesics such as pregabalin and gabapentinoids – all of which are addictive and potentially dangerous drugs.
Mr Jones said: "Let’s supply cannabis in controlled conditions and see if offences reduce. The aim of the game is to make prisons safer. If they’re serious about reducing violence in prisons they should be addressing the causes and that’s psychoactive substances. Plus there’s a whole range of issues that cannabis would be geared to reduce the risk of.”
The idea of trialling free cannabis in prisons was floated in 2018 by the pharmacologist Dr Stephanie Sharp.
She said that leaving prisoners to smoke spice was “condemning them to death” and that allowing then to smoke cannabis would be “much safer”.
Mr Jones has also called for cannabis to be regulated to cut out organised crime and allow people to grow a limited amount of it for their own personal use.
“It is a nonsense to criminalise people who take cannabis for recreational use and cause no harm to anybody else.
“The best way to reduce the role of organised crime in the supply of drugs is to put it in commercial hands and to price it appropriately so people don't need to go to the illegal market.
“Commercial organisations have taken over the medicinal cannabis market and are selling prescriptions at a vast cost even though it is cheap to grow.
“My view is that people should be allowed to grow a limited number of cannabis plants for their own use.”