"We recognize the incredible work our pharmacists and technicians do every day and have taken a number of steps in our pharmacies to ensure that our teams can concentrate on providing optimal patient care," it said.

CVS said it was not seeing any "unusual activity" connected to store closures or walkouts and that it has made changes to address some of the past complaints.

"In response to recent feedback from our pharmacy teams, we're making targeted investments to address their key concerns, including enabling teams to schedule additional support as needed, enhancing pharmacist and technician recruitment and hiring, and strengthening pharmacy technician training," the company said.

Michael Hogue, CEO of the American Pharmacists Association, said his group supports the employees who walked out. "For far too long, employers have made the situation worse than it needed to be," he said, adding that quotas requiring pharmacists to fill a certain number of prescriptions or administer large numbers of vaccinations are damaging their relationships with patients.

The current walkout is just the latest example of growing labor unrest. More than a dozen CVS pharmacies in the Kansas City area closed during a September walkout. Roughly the same number of Walgreens stores nationwide closed during a walkout in early October.

Related: 'Patients before profits': Why Walgreens (and CVS) pharmacists are walking off the job

In an attempt to cut costs following consolidation in the pharmacy industry, large companies have not employed enough pharmacists and technicians at their stores, leaving staffs overburdened, said John August, director of health-care labor relations at Cornell University. "The pandemic just caused so much additional work and stress that the turnover rates, which were already very high, just became extremely epidemic," he said.

Since the vast majority of pharmacists and technicians from Walgreens and CVS do not have union representation, the pharmacists' actions have caught the eye of organized labor, such as the United Food & Commercial Workers union, which represents some employees at CVS, Walgreens and Rite Aid.

"UFCW members and staff have been communicating with many of the CVS and Walgreens workers who have been taking actions to stand up for their rights on the job," the union's organizing director Dave Young said in a Tuesday statement. "Where workers struggle, we stand ready to assist."