Enlarge (credit: Getty | Spauln)

The dose makes the medicine—and for many critical prescription drugs, the dose depends on a patient's body weight. Usually, this is not a problem; weight changes large enough to significantly affect dosages often occur gradually, over periods long enough for doctors to notice and adjust prescriptions. But, in the era of new weight loss drugs, that may no longer be the case.

In a cautionary tale published Monday in JAMA Internal Medicine, researchers at the University of Colorado reported the case of a man who lost nearly 30 percent of his body weight in a six-month period using a new weight loss drug. Then, he showed up at an emergency department with heart palpitations, excessive sweating, confusion, fever, and hand tremors. Tests indicated the man had atrial fibrillation, an irregular heart rhythm that can lead to heart failure and stroke without treatment.

The 62-year-old had no history of atrial fibrillation, but he had previously been diagnosed with obesity, Type 1 diabetes, and hypothyroidism (a condition in which the thyroid gland does not produce enough thyroid hormone). For his hypothyroidism, he took levothyroxine, a synthetic thyroid hormone that is dosed by weight.

Enlarge / Packaging for Wegovy, manufactured by Novo Nordisk, is seen in this illustration photo. (credit: Getty | Jakub Porzycki)

With the debut of remarkably effective weight-loss drugs, America's high obesity rate and its uniquely astronomical prescription drug pricing appear to be set on a catastrophic collision course—one that threatens to "bankrupt our entire health care system," according to a new Senate report that modeled the economic impact of the drugs in different uptake scenarios.

If just half of the adults in the US with obesity start taking a new weight-loss drug, such as Wegovy, the collective cost would total an estimated $411 billion per year, the analysis found. That's more than the $406 billion Americans spent in 2022 on all prescription drugs combined.

While the bulk of the spending on weight-loss drugs will occur in the commercial market—which could easily lead to spikes in health insurance premiums—taxpayer-funded Medicare and Medicaid programs will also see an extraordinary financial burden. In the scenario that half of adults with obesity go on the drug, the cost to those federal programs would total $166 billion per year, rivaling the programs' total 2022 drug costs of $175 billion.