• A new study concludes that the diuretic drug chlorthalidone may cause more side effects than hydrochlorothiazide, another commonly prescribed medication for high blood pressure.
  • Researchers say chlorthalidone can produce dangerously low levels of potassium, a nutrient that’s critical to controlling heart and muscle function.
  • However, experts say the research on diuretics is conflicting, so people shouldn’t switch drugs until more studies are done.

Medications such as diuretics can be key for people with high blood pressure.

By causing you to urinate more often, the drugs can help remove excess water and salt from your body.

In turn, diuretics can help lower blood pressure and reduce the risk of complications such as heart attacks or strokes.

But these drugs can cause some milder complications of their own.

Which diuretics are most effective at reducing high blood pressure and which are most likely to have side effects has been an ongoing area of study.

A 2017 report by the American College of Cardiology listed chlorthalidone as the preferred diuretic due to its long lasting power and its reduction of cardiovascular disease risk.

However, a study published this week has concluded that chlorthalidone may cause more side effects than hydrochlorothiazide, another medication commonly prescribed for hypertension.

Experts say it’s too soon to revisit recommendations and that people currently using one drug or the other likely don’t need to worry about switching medications.

They agree the new findings raise questions that warrant more research.

What the study showed

The new study found that people who took chlorthalidone had a nearly three times higher risk of developing dangerously low levels of potassium, a nutrient that’s critical to controlling heart and muscle function.

Low potassium levels can lead to an abnormal heartbeat, confusion, kidney failure, and type 2 diabetes.

By looking at 17 years of data on more than 730,000 people with hypertension, researchers found that 6 percent of those treated with chlorthalidone experienced low levels of potassium, known as hypokalemia.

By contrast, almost 2 percent of people treated with hydrochlorothiazide did.

The researchers also said they found that the two drugs were similarly effective in preventing complications from hypertension, such as heart attacks and strokes.

Dr. George Hripcsak, MS, the lead author of the new study and chair of the department of biomedical informatics at Columbia University’s Vagelos College of Physicians and Surgeons in New York, noted that while the current guidelines include both drugs, they “talk about chlorthalidone being preferred.”

That’s because that drug is longer acting, he told Healthline, and because recent trials have shown an improvement in heart health using chlorthalidone.

He does note that the research doesn’t mean that hydrochlorothiazide would not have also worked.

“I would amend that (guideline) note, stating that observational trials do show greater side effects in chlorthalidone. Further study is being done,” he said.

What the research means

For now, it’s likely too soon to update any guidelines.

“The balance of existing data still supports the use of chlorthalidone for hypertension treatment,” said Dr. Gregg C. Fonarow, chief of the University of California, Los Angeles division of cardiology and an expert with the American Heart Association. “However, as additional randomized clinical trial data become available, the guidelines may be updated.”

As for people who are taking one of these drugs for hypertension, the takeaway appears to be to just keeping talking with their doctor to ensure that the drugs are working for them and they’re not having severe side effects.

“It is critical for individuals with high blood pressure to achieve and maintain evidence-based, guideline-recommended blood pressure goals with a well-tolerated medication regimen together with lifestyle modification,” Fonarow told Healthline.

That may mean sticking with chlorthalidone.

“If chlorthalidone is working, I personally would just make sure that the doctor is careful monitoring electrolytes like potassium and kidney function,” Hripcsak said.

Dr. Eugene Yang, MS, FAAC, a professor of medicine at the University of Washington and chair-elect of the American College of Cardiology’s Prevention of Cardiovascular Disease Committee, is concerned that people will see news stories about a study like this and call up their doctor asking if they should switch medications.

Yang told Healthline he even replied to a tweet from a study co-author with those concerns.

Yang said that the author agreed and clarified that the study’s findings would only be intended to help in making decisions about people who are starting hypertension medications, not those already taking one drug or the other.

“The way I would look at this study as a clinician is we don’t need to tell patients that they need to come off of these medications,” Yang said.

There are some situations, such as if a person is showing signs of diabetes or kidney failure, where switching might be considered, he noted, “But otherwise I don’t think we should treat this as a study that means we change how we treat patients.”

Yang added, though, that the study “does give us some reassurance” that for a person with hypertension who’s starting treatment, hydrochlorothiazide may be OK, “But we don’t know until we have randomized trial data.”

Future research

That data could come from a study by the Veterans Administration that’s directly comparing the two drugs.

This study is considered important because both the new study and a 2013 study made note of a possible risk of low potassium levels tied to chlorthalidone.

Hripcsak noted that other studies have found no increased significant risk of side effects between the two drugs and that chlorthalidone was more effective in reducing high blood pressure.

“So the evidence was conflicting,” he said. “A randomized trial is currently in progress, but we won’t have the results for some time.”

The Veterans Administration’s randomized trial is slated to include 13,500 people with hypertension, with results expected in 2022.

“The most compelling randomized clinical trials demonstrating cardiovascular event reductions have utilized chlorthalidone,” said Fonarow.

But he noted that hydrochlorothiazide is more commonly prescribed in the United States.

The new study, he pointed out, is, like the 2013 one, “observational,” meaning it observed outcomes in the real world where other variables beyond the drugs may influence things such as heart health and potassium levels.

Given those limitations, Fonarow said, “These results are hypothesis-generating rather than definitive, and prospective randomized trials are needed.”

That’s where the Veterans Administration study may prove important.

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