May 28 (Reuters) - As millions seek access to weight-loss drugs from Novo Nordisk and Eli Lilly , increasing supplies, possible wider usage and a growing number of would-be rivals are leading some experts to raise annual global sales forecasts for the treatments to about $150 billion by the early 2030s.

A year ago, top sales estimates were in the $100 billion range.

"It is very unusual to have a medicine that is capturing the imagination of millions of people," said Michael Kleinrock, senior research director at healthcare analytics firm IQVIA Institute for Data Science.

Most insurers do not cover the new therapies with low co-payments, but an unprecedented percentage of people are paying themselves or with coupons from drug manufacturers, he said.

Global spending on obesity medications totaled $24 billion last year, IQVIA estimated in its latest five-year outlook, and could reach $131 billion by 2028. That 27% annual growth estimate compares with a prior projection of 13%.

Without insurance coverage expansion, IQVIA put the low end of global obesity-drug spending at $39 billion in 2028 versus a more likely $74 billion.

Kleinrock said reaching $131 billion will also depend on how long patients stay on a drug, whether the medications are used to treat other diseases, or even development of new direct-to-consumer sales models.

Shortages that capped sales in 2023 are being resolved, he said, although sales are still limited in large part only by manufacturing capacity.

Supplies of both Novo's Wegovy and Lilly's Zepbound remain constrained, but the companies have been increasing production.

'CONSUMER DEMAND, UNMET MEDICAL NEED'

BMO Capital Markets now estimates annual weight-loss drug sales reaching $150 billion by 2033, up from a year-ago forecast of over $100 billion by the early 2030s.

Leerink forecasts annual sales of $158 billion by 2032.

Analysts cite recent data showing the self-injected drugs help stave off costly emergencies like heart attack and stroke or treat chronic conditions like sleep apnea, supporting the case for employers and insurers to pay for them.

"There's consumer demand and the unmet medical need," said David Song, portfolio manager of the Tema Obesity & Cardiometabolic ETF. "A 100 million plus Americans are obese, and even more are overweight. Worldwide, there are estimates out there of close to a billion who are obese."

Sales of the new medications, which have U.S. list prices of over $1,000 a month, have lifted Lilly and Novo into the ranks of the world's most valuable companies. Shares in Lilly are up 36% so far this year, while Novo has gained 33%.

Rivals see room for treatments that are more convenient, offer better weight loss or promise additional health benefits. Some seek to improve durability or quality of weight loss by distinguishing between fat and lean body mass.

Lilly and Novo are also developing next-generation compounds.

Over 80 experimental obesity drugs have reached the human testing stage, according to IQVIA.

The pipeline is dominated by drugs that, like Wegovy and Zepbound, mimic an intestinal hormone called GLP-1, either on their own, or in combination with compounds that target a second hormone called GIP, such as Amgen's Maritide. Amgen expects mid-stage trial results late this year.

Other experimental drugs, like Lilly's retatrutide, target the blood sugar-regulating hormone glucagon in addition to GLP-1, GIP or both.

The third largest category includes drugs like Novo's amycretin, which in addition to binding to GLP-1, targets a hormone called amylin in the pancreas that affects hunger.

"There will be price competition" as new players enter the market, Song said, but the bull case is that access will widen and higher volume will offset price erosion. (Reporting By Deena Beasley in Los Angeles; additional reporting by Patrick Wingrove in New York; Editing by Caroline Humer and Bill Berkrot)