It comes after the prisons service launched a new strategy to tackle the flood of narcotics finding their way into jails, which has been blamed for an increase in violence and self-harm.
Ministry of Justice figures show 124 adjudications – mini court hearings which take place when a prisoner is accused of rule-breaking – were recorded for drugs-related infractions at HMP Preston in 2019.
Of these, 58 (47 per cent) resulted in a guilty verdict. That was a drop of 30 per cent from 2018, when there were 83 guilty verdicts.
Overall, 2,017 adjudications took place during the year, meaning drugs featured in six per cent of cases.
Other reasons include assaults, threats or abusive behaviour, and disobeying orders.
The drop at Preston mirrored the picture across England and Wales, with the number of adjudications leading to a guilty verdict falling by more than a quarter during the same period, from 11,523 to 8,319.
Charity the Prison Reform Trust urged caution in interpreting the figures, arguing that reliable measures of drug use in prison are "notoriously difficult to design".
Director Peter Dawson said: "These figures on their own cannot tell the whole story.
"But it is reasonable to expect that a balanced strategy, which both restricts supply and reduces demand, will bear down on illicit drug use.
"The prison service has published such a strategy – everything now depends on implementing all of its elements, the carrot as well as the stick."
The charity had previously called for greater investment to tackle the root causes of drug use in prison, which it said fuelled violence and self-harm, and spilled over to families outside.
During the year, 78 punishments were handed out for drug offences at Preston – 1.3 punishments per offence.
The most common sanctions,used 23 times each, were confinement to the prisoner's cell and the forfeiture of privileges, such as the prisoner's access to a TV, extra visitors, or permission to wear their own clothes.
The misuse of drugs in prisons was identified as "one of the biggest challenges facing our criminal justice system today" by the MoJ and HM Prisons and Probation Service, when they set out their strategy to tackle the issue in April last year.
The report acknowledged the "significant" scale of the problem, with the rate of positive results for random drug tests increasing by 50 per cent between 2012-13 and 2017-18.
An MoJ spokesman said the Government is investing £100 million to bolster prison security, clamping down on drugs and weapons that fuel violence.
He said: "Illicit substances pose huge challenges in our prisons which is why we have brought in airport-style security – including x-ray body scanners – to stop them getting in.
"Extra training is making jails safer for offenders and staff, and we work closely with healthcare providers to ensure prisoners have the support they need to live drug-free upon release."