Brenda Henninger takes potassium citrate for her kidneys, and she knew something was wrong when she opened her refill.

"It was not the right pill,” Henninger said. “It just did not look right."

It was potassium chloride, a similar-looking white pill with a similar name.

When Henninger questioned the pharmacy, she said the pharmacy manager asked her, " 'What did you get?’ I told him the potassium chloride instead of the potassium citrate. And he said to me, 'Well, potassium is potassium.' "

But Henninger later learned from another pharmacist that wrong potassium could have killed her. "Probably a heart attack or a stroke," she said.

Prescription pills are so small, but so powerful. Patients are faced with a big question each time they fill a prescription: Are the pills in the bottle going to make you better? Or hurt you?

"I was concerned we were going to kill somebody,” said Joe Zorek, who was a pharmacist with CVS/pharmacy for more than 30 years. Zorek recently filed a whistleblower lawsuit against his company, claiming CVS retaliated against him when complained about changes he said the company started making about five years ago.

"Everything was being gauged, was being checked. Time constraints," Zorek said. "22 seconds to answer a phone. You were getting 15 minutes to fill a prescription."

At the same time, Zorek said his pharmacy staff was cut 20 percent, leaving less time to fill prescriptions and counsel patients.

The News 4 I-Team concealed the identity of another pharmacist currently employed with CVS/pharmacy who, afraid of getting fired, described the company’s time requirements and mistakes.

"I would say there's at least one a day if not more, and they don't get caught until they get refilled," the pharmacist said.

We obtained recent copies of internal CVS/pharmacy measurements these whistleblowers told us the company sends its pharmacists each week. Percentage grades are given for the number of flu shots administered, how quickly voicemails are checked and how long it takes to fill prescriptions.

"Rushing through them, you're going to miss stuff,” the second pharmacist said. “You know, you miss one word in the directions and that can change something drastically."

Zorek had a different name for it. “It's McPharmacy. You know, get it done as quickly as you possibly can. And that becomes scary after a time."

Dr. Carmine Catizone at the National Association of Boards of Pharmacy said, “We've heard the complaints about the large chains and how they're morphing or how they resemble fast food restaurants."

His group represents government agencies in charge of ensuring pharmacy safety and said no one is keeping track of mistakes because large chains refuse to turn over the data.

But he explained, that’s the only way to fix the problem. "The state boards of pharmacy [need] to have the legal authority to mandate the reporting of medication errors."

Until then, the main way mistakes are discovered is when a patient complains and a state takes disciplinary action.

Like a Virginia case we found, where a CVS pharmacist dispensed a bipolar disorder drug to a child instead of the prescribed blood pressure medication.

In New Jersey, the Attorney General fined CVS $650,000 last year after it found multiple stores made medication mistakes, including as many as 50 children receiving the breast cancer drug Tamoxifen instead of fluoride pills.

In a statement, CVS told the News4 I-Team:

“The health and well-being of our patients is our number one priority… Like other companies, CVS/pharmacy measures the quality and effectiveness of the services we provide in our pharmacies to ensure we are meeting our customers’ expectations. Our systems are designed to help our pharmacists manage and prioritize their work to best serve their patients… Every prescription dispensed at CVS/pharmacy undergoes a multi-step review by a pharmacist prior to being dispensed to a patient. Prescription errors are a very rare occurrence, but when they happen we do everything we can to take care of the patient's needs… We also examine how an error may have occurred as a critical part of preventing errors in the future, so we encourage our pharmacy teams to report such incidents.”

Experts say you should always double-check the drug you received by researching what the drugs looks like online. Also, always check the dosage.

If something doesn't look right - call your doctor.