Two SPAR stores in Cardiff are using breathalysers on customers they suspect are drunk before allowing them to buy booze.
The equipment is being used as part of a police crackdown on street drinking and anti-social behaviour in the city centre, dubbed Operation Purple Ash.
But the measure is not going down well with some customers, with shop workers complaining they have received abuse.
Suhail Ahmed, the decade-long owner of SPAR in St Mary Street, told Sky News: "In most situations I don't think a retailer would need one, but over the summer Cardiff attracted a lot of street drinkers. The supermarkets provide very cheap alcohol.
"The law states that if someone is intoxicated, you're not allowed to serve them. The breathalyser is a tool and it provides clarification. We're a busy store and we use it when we have to, mostly on the street drinkers.
"I've seen some people moaning that it's unhygienic but you don't actually have to touch it - you can breathe into it easily without your lips touching it."
Another SPAR owner in the city, 40-year-old Rohat Hamed, said: "We've used it hundreds of times already - we just grab it and if anybody who tries to buy alcohol looks drunk we tell them to blow into it.
"Some people have given us abuse when we use it because they must think that they're above the law."
Dr Kevin Smith, a senior lecturer at Cardiff University's School of Social Sciences, has questioned the stores' use of the breathalysers.
He told Sky News that such measures were not the correct response to social issues in the city, including the abundance of rough sleepers.
He said: "If this is a way for SPAR owners to protect their employees from potential harm then that seems like a morally good response, but if it's something that's come from the police as part of a larger reaction to what they feel is antisocial behaviour, then that is something that is beyond the remit of a SPAR employee.
"You'll have all these Christmas dos coming up soon and if my friends and I went out in our suits and ties and want a can from the local SPAR, honestly, they are not going to breathalyse us. But they are going to breathalyse the people they think they fit the profile.
"We have suffered under a lot of austerity and it's trickled down. Rough sleeping is an everyday activity that people just accept, when it should be unacceptable. This larger, organised police response almost feels like the criminalisation of victims rather than a supportive way of dealing with it."
But South Wales Police's divisional commander for Cardiff, chief superintendent Belinda Davies, said: "Complaints from visitors, residents and businesses around the number of people begging and undertaking alcohol-related anti-social behaviour within the city centre have increased in recent months.
"Such behaviour can prove intimidating, unpleasant and unwelcoming to those visiting or working in the area.
"Our action is intended to deal with persistent beggars and street drinkers, but also giving us the opportunity to engage with some of the more vulnerable members of the community, working with our partners to signpost a variety of help and support services available to them."
Eighteen people have also been arrested since the operation began, some for being drunk and disorderly.