Update: This post has been updated from the orginal post on February 11.

Two drugs approved in 2012 to treat obesity have hardly become blockbusters used by millions looking to shed a few pounds. That’s partly because of the cost and spotty insurance coverage: a month’s supply is about $160 to $200. But it may also be due to the drugs’ modest effectiveness and safety concerns, according to two Dartmouth researchers who wrote a column warning about the drugs’ unknown health risks in the February 10 issue of JAMA Internal Medicine.

Insurance plans more readily reimburse for far more expensive weight loss surgeries, such as gastric bypass, to reverse obesity than they do for lorcaserin (Belviq) or phentermine-topiramate (Qysmia), the two new drugs. But that may not be a bad thing, until more is known about their long-term side effects, the doctors argue.

Advertisement - Continue Reading Below

“Neither medication is marketed in Europe because of safety concerns,” wrote column authors Dr. Steven Woloshin and Dr. Lisa Schwartz, family medicine physicians at the Dartmouth Institute for Health Policy and Clinical Practice in Lebanon, N.H. “The drugs have been associated with serious harms.”

They’re referring to uncommon side effects seen in clinical trials that are listed on the drugs’ labels, including memory loss, attention difficulties, language problems, and depression. Both drugs also cause low blood sugar in those taking medications for type 2 diabetes—a condition that many obese people have.

“People taking medicines like Belviq have had heart valve problems,” reads the manufacturer’s website. “Belviq may slow your thinking. You should not drive a car or operate heavy equipment until you know how Belviq affects you.”

Really? That makes it kind of hard to get from place to place when you’re taking the drug.

Qsymia’s website warns about the drug’s potential to increase heart rate, trigger suicidal thoughts and actions, and cause serious eye problems, including a sudden decrease in vision. Less serious effects include mood changes and trouble sleeping.

In deciding to sign off on the two weight loss drugs—the first approved in more than a decade—the US Food and Drug Administration determined that the chronic health problems associated with being obese outweigh risks associated with the drugs. But the agency also required that the manufacturers continue to conduct safety trials to learn more about the side effects.

Woloshin and Schwartz wrote that these “safety trials are behind schedule” and “until there is more convincing evidence about the cardiovascular safety of these drugs, physicians and patients should approach them cautiously.”

Eisai Inc., maker of Belviq, announced on Monday that they began recruiting 12,000 patients to take Belviq or a placebo for a five-year safety study to determine any increased risks to the heart that will “satisfy the postmarketing requirement from the FDA.”

In the meantime, patients may want to heed the Dartmouth researchers’ advice: Those who try either drug and who do not lose at least 5 percent of their body weight within 12 weeks should stop taking it. That’s advice also provided by the manufacturers, but doctors may not be heeding it.