Illegal drugs should be decriminalised, delegates at the Plaid Cymru conference have said.
Party members called the war on drugs an "unmitigated failure" and said criminalising those with an addiction does "nothing to help them turn their lives around".
Activists voted for decriminalisation to become party policy on Friday.
The party has four MPs in Westminster, where drugs policy for England and Wales is controlled.
Portugal decriminalised drugs in 2001.
A conference motion, raised by Plaid Cymru's youth movement Plaid Ifanc, called on the Home Office to cleanse the criminal records of those cautioned or convicted of drugs possession where there were no aggravating features.
"We do not advocate using these consuming these substances," the motion said, adding "addiction is an illness and not a crime".
It said that "criminalising those that have an addiction... will do nothing to help them turn their lives around".
Plaid Ifanc's Carmen Smith said the "current hard-line approach to drugs" criminalises "otherwise ordinary citizens, medicinal and recreational drug users who do no harm to others".
"Far more importantly, however, the current approach to drug use fails the most vulnerable," she said, citing recent claims by the Cardiff Huggard Centre that Spice use was at epidemic levels.
Plaid Cymru North Wales Police and Crime Commissioner Arfon Jones said there were three arguments against decriminalisation: that it would increase the use of drugs, promote drugs tourism and increase criminality associated with drugs.
None of this had occurred in Portugal following decriminalisation, he said.
But one delegate said decriminalisation was a "first step by people who want to see legislation" and that people who take soft drugs in their 20s suffer mental health problems in middle age.
He claimed he was "bullied online" because of his stance against decriminalisation.
The motion calls for all police and crime commissioners to introduce a diversionary scheme to divert minor possession of drugs away from criminal justice agencies and into public health and education.
Plaid Cymru has two Welsh PCCs - Arfon Jones and Dafydd Llewellyn for Dyfed Powys.