The photocopying of inmate mail at a prison in a bid to prevent drug-soaked letters being smuggled in has reduced on-site substance abuse, a report said.
A prison watchdog inspection found the "intrusive" move at HMP Humber caused prisoners "much anger", but had lowered the use of new psychoactive substances.
A survey found 63% of inmates obtained illegal drugs easily, with a third developing a problem at the prison.
The Ministry of Justice said a "robust" drug strategy was dealing with usage.
HM Chief Inspector of Prisons visited the category C prison, near Everthorpe, East Yorkshire, in late 2017 after its previous report found the availability and use of illegal drugs was too high.
It discovered 38% of inmates tested positive for drugs, including new psychoactive substances (NPS), during random tests over the previous six months.
NPS-related incidents had reduced from "a very high number" in early 2017, with the prison attributing the decrease to the photocopying of all prisoner mail.
The watchdog called the move a "justifiable short-term response", but its ongoing value had not recently been reviewed.
Its inspection also found high levels of victimisation, intimidation and violence, with much of it "underpinned by a pervasive drug culture".
It noted the prison was well led, staff appeared committed and there seemed to be a "new-found and growing confidence about its future".
Support for prisoners with drug problems was described as commendable, despite staffing shortages.
Michael Spurr, head of the prison service, said: "I'm pleased that the inspectorate have acknowledged the progress made at Humber over the last 12 months.
"A robust strategy is in place to tackle illicit drug use, including a new specialist intelligence unit which will work closely with police colleagues to target drug supplies."
HMP Humber, which holds more than 1,000 inmates, was created when a secure corridor was built to connect Wolds and Everthorpe prisons.