A benefit system shake-up is having a "devastating impact" on people in Flintshire, a council boss has warned.
It was the first Welsh county to see the introduction of universal credit.
Flintshire's head of revenue collection David Barnes said average rent arrears have risen four-fold since the money is now paid to claimants, not landlords.
The Department for Work and Pensions (DWP) said the proportion of people with arrears fell by a third after four months on universal credit.
The UK government has said it wants to simplify the welfare system by replacing six different benefits with a single payment.
It has faced criticism for delays in payments, and for a change which means money for rent is given to the claimant rather than directly to landlords.
Universal credit claimants in Flintshire now faced average rent arrears of £1,473 per person, compared to £334 under the old system, according to the Local Democracy Reporting Service.
'Head in the sand'
Mr Barnes told a Flintshire council scrutiny committee on Wednesday the last two years had been his "most challenging" in 32 years working in debt collection.
"I came across a case last week of a vulnerable tenant who had drugs and alcohol dependency," he said.
"They admitted they'd buried their head in the sand, and their first payment [of universal credit] went to pay drug debts.
"The reality is not all tenants can budget properly and they fall into arrears."
In response, the DWP said: "Our research shows that many people join universal credit (UC) with pre-existing arrears, but the proportion of people with arrears falls by a third after four months in UC.
"Managed payments to landlords are available as part of the alternative payment arrangements in universal credit, to minimise the risk of claimants failing to pay their rent.
"And we are rolling out the Universal Credit landlord portal to social landlords, which is helping us target support for vulnerable people."