A shoplifter slashed a 63-year-old security guard in the face late Monday at an Upper East Side CVS — where workers are told just to be “extra friendly’’ in hopes potential shoplifters will think twice about stealing.

The armed thief was swiping an electric razor and other shaving items – including blades and creams – from a shelf and stashing them in his backpack in the convenience store on East 86th Street near Second Avenue around 11:30 p.m. when the guard tried to stop him, police said. 

The suspect whipped out a knife and slashed the employee just below the left eye, authorities said. 

The wounded worker was taken to Weill Cornell Medical Center with non-life-threatening injuries, cops said. 

The thief got away with the loot, worth nearly $400, police said.

He was still on the loose Tuesday morning. 

Store workers told The Post they knew little about what happened the night before. 

But a staffer who did not want to be identified said employees aren’t supposed to physically interfere with anyone they spot stealing. 

“We do get a lot [of shoplifters],” the worker said. “However, there’s a rule that we can’t put our hands on the shoplifters. 

“We just have to be extra friendly to make sure they didn’t steal.” 

The staffer explained that “in general we’re supposed to be friendly to customers.

“But if we think someone is stealing, the approach is to be annoyingly friendly, like ‘Do you need help with this? Do you need help with that?’ So they are aware that we see them.

“Especially around this time of year, there’s more of them,” he added. “But it happens more at night.” 

Another worker said the staff faces a dilemma any time shoplifting happens. 

“I’m extremely curious [about] the company’s take on it, because if we do something, we get fired,” he said. “If we don’t do something, it keeps happening. They could be stealing or go[ing] up to a customer, and we’re not allowed to step in.” 

He said he is not “expecting” assaults like this to happen on the job, but “growing up in the city, things like that happen here.” 

Reached by The Post by phone, the injured guard confirmed he was the “overnight supervisor” at the CVS and that he was “involved in the incident last night.” 

He then said he’d call a reporter back before hanging up.