More than a kilo (2.2lbs) of drugs previously known as "legal highs" and dozens of mobile phones have been seized at a prison in a single month.
Inspectors also revealed they were told "horrific" stories concerning the possible effects of new psychoactive substances (NPS) - including one young man who had blinded himself.
The disclosures were contained in a report by the prisons watchdog on HMP Lindholme, a category C prison near Doncaster, South Yorkshire, which holds just over 1,000 adult male inmates.
Links with crime agencies were "impressive" and resulted in some successful operations, the report said.
It went on: "These operations had led to the capture of large hauls of contraband, which in a single month had included over a kilo of NPS, 67 mobile phones, 145 Sim cards, steroid vials and tablets, and injecting equipment, as well as finds of heroin, cocaine and other drugs."
HM Inspectorate of Prisons said the influx of drugs was "destabilising" the establishment.
The prison seized considerable amounts of illegal substances, but nearly two-thirds of prisoners told inspectors it was easy to obtain illegal drugs.
The report added: "The stories we were told, concerning the possible effects that NPS was having on individuals, including one young man who had literally blinded himself, were nothing short of horrific."
No further details of the incident were given.
Grave concerns have been raised about the impact of the drugs - known as legal highs before they were made the subject of a blanket ban earlier this year - across the prison estate.
Figures show NPS were linked to the deaths of at least 39 prisoners in two years.
Overall, the watchdog said serious concerns still need addressing at HMP Lindholme, but its deterioration had been halted and work, training and education had improved.
Chief Inspector of Prisons Peter Clarke said: "This is a mixed report.
"That said, it was clear that the prison was led by a focused and committed governor and management team, aided by a much better approach now being adopted by staff. Lindholme was a recovering prison and we were confident that improvement could continue.
"The priorities were clear to us: a robust strategy to stop NPS and, linked to that, to reduce violence; significant improvements in offender management and proper arrangements to provide resettlement services."
Michael Spurr, chief executive of the National Offender Management Service, said: "Lindholme provides a purposeful regime for prisoners with a focus on rehabilitation, which is commendable.
"Their efforts have been undermined by the illegal supply of new psychoactive substances and the violence it encourages. The governor is working hard with police colleagues to tackle this threat - and to generally improve safety.
"I share the Chief Inspector's confidence that, despite the difficult challenges it faces, Lindholme is an improving prison moving firmly in the right direction."