Over 17% Haverfordwest of drinkers tested for contact with drugs have produced a positive result.
Last Friday night police hit the town with an Ion scanner and tested revellers in the town's Castle pub for drugs.
Police took swabs from the hands of 282 drinkers. Nearly a fifth of these showed that the person had been in contact with drugs, or had been in contact with something else that had traces of drugs on it.
"The Ion scanner tells you if any contact has been made with drugs or other materials with traces of drugs," said police spokesman Ceri Doyle.
"It does not tell you how much of the drug the person has been in contact with. It is a good starting point for further investigations."
Of those tested, between 8pm and midnight, 49 showed positive traces of drugs, nearly a fifth of the sample.
One person was found in possession of cocaine and arrested.
“The Ion scanner can give us evidence which we can then use to search a person," said Inspector Gareth Thomas.
“We are pleased that only one person of all we swabbed was found to be in possession of illegal substances.
“I would like to thank The Castle and the people out on Friday night for their co-operation in this operation which will be repeated.”
Friday's operation is just one of the events taking place as part of Pembrokeshire police's Not in my Neighbourhood week, which officially kicked off yesterday (Monday).
Throughout the week officers will be going into schools talking to young people about anti-social behaviour, how to stay safe at Halloween and the dangers of fireworks. The neighbourhood policing teams will also be out in supermarkets talking about the issues.
Another part of the week's activities focus on vehicle crime: Traffic wardens will be out in force and the DVLA will be targeting illegal vehicles.
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