Dr. Robert H. Lustig is an endocrinologist, a professor emeritus of pediatrics at the University of California, San Francisco, and an author of best-selling books on obesity.

He is absolutely not — despite what you might see and hear on Facebook — hawking “liquid pearls” with dubious claims about weight loss. “No injections, no surgery, just results,” he appears to say in one post.

Instead, someone has used artificial intelligence to make a video that imitates him and his voice — all without his knowledge, let alone consent.

The posts are part of a global surge of frauds hijacking the online personas of prominent medical professionals to sell unproven health products or simply to swindle gullible customers, according to the doctors, government officials and researchers who have tracked the problem.

While health care has long attracted quackery, A.I. tools developed by Big Tech are enabling the people behind these impersonations to reach millions online — and to profit from them. The result is seeding disinformation, undermining trust in the profession and potentially endangering patients.

Even if the products are not dangerous, selling useless supplements can raise false hopes among people who should be getting the medical treatment they need.