Sharri Shaw walked out of the CVS on Vermont Avenue in South Los Angeles in 2019 believing she had a prescription for the pain reliever acetaminophen.

Instead the bottle held a medicine to treat high blood pressure, a problem she did not have.

Shaw began taking the pills, not learning of the mistake until six days later when a CVS employee arrived at her home, according to a lawsuit she filed last year.

The employee told her not to take the tablets, the lawsuit said, before leaving the correct prescription at her door. The mistake, she said, left her in shock.

Shaw’s experience is far from an isolated event. California pharmacies make an estimated 5 million errors every year, according to California’s Board of Pharmacy.

Officials at the regulatory board say they can only estimate the number of errors because pharmacies are not required to report them.

Most of the mistakes that California officials have discovered, according to citations issued by the board and reviewed by The Los Angeles Times, occurred at chain pharmacies such as CVS and Walgreens, where a pharmacist may fill hundreds of prescriptions during a shift, while juggling other tasks such as giving vaccinations, calling doctors’ offices to confirm prescriptions and working the drive-through.

Christopher Adkins, a pharmacist who worked at CVS, and then at Vons pharmacies until March, said that management policies at the big chains have resulted in understaffed stores and overworked staff.

“At this point it’s completely unsafe,” he said.

Adkins now works at an independent pharmacy company in Los Angeles. He isn’t the only pharmacist worried that heavy workloads and distractions are leading to errors.

In a survey of California licensed pharmacists in 2021, 91% of those working at chain pharmacies said staffing wasn’t high enough to provide patients adequate care.

While the pharmacy board requires pharmacies to document errors internally, inform patients about mistakes in certain cases and learn how to prevent them from occurring again, only 62% of chain pharmacists said stores were following those rules, according to the survey.

Medication errors can harm patients, sometimes seriously.

One patient took prednisone, a powerful steroid, for 89 days after a Walgreens pharmacist in Santa Clara confused the drug with Prilosec, the heartburn drug that had actually been prescribed, according to a citation issued in June 2022. Research has shown that Prednisone can decrease bone density within weeks of starting the drug, increasing the risk of fractures.

In Inglewood, a pregnant patient suffered a fall after she was given two drugs prescribed to another customer by a pharmacist at the CVS on Market Street, according to an August 2021 citation.

Some patients have continued to take and refill the prescriptions without knowing they received the wrong drug.

The state’s pharmacy board says it investigates any report of an error that it receives from consumers or others. If the investigation finds that regulations were violated, the board can issue citations and fines and possibly take away the pharmacist’s or pharmacy’s state license.

CVS and Walgreens declined to make executives available for interviews but sent written statements saying that the errors were rare. Rite Aid did not respond to messages.