The maker of Ozempic and Wegovy is suing a rival firm for selling what it says are "unsafe, knock-off" versions of its weight-loss drugs in the US.

Danish company Novo Nordisk asked US courts on Monday to ban Hims & Hers' range of weight-loss pills and injections, which it says are not approved by US authorities and infringe on its patent.

The legal drama began on Friday after Hims & Hers launched a new weight-loss pill, leading to an initial threat from Novo Nordisk.

Over the weekend, Hims & Hers said it would stop selling the pill. On Monday, its share price slumped as it called Novo Nordisk's decision to press ahead with the lawsuit a "blatant attack".

Hims & Hers shares closed down 16% on Monday, while Novo Nordisk's share price nudged up early on Tuesday.

Weight-loss drugs like Novo Nordisk's Ozempic and Wegovy and Eli Lilly's Zepbound and Mounjaro have soared in popularity over the last few years.

Patent expiry has also led to rise of low-cost copycat weight-loss drugs, with warnings about the safety of some of them.

Novo Nordisk said on Monday its action against Hims & Hers aims to "protect public health and defend the scientific innovations that deliver better health outcomes to Americans".

Hims & Hers said in response it has "a long history of providing safe access to personalized healthcare to millions of Americans", and accused Novo Nordisk of "weaponizing the US judicial system to limit consumer choice".

'Fighting back'

Kerry Fulford, a pharmaceutical analyst at Berenberg, told the BBC that this was the "first move Novo Nordisk is making to clamp down on compounding".

Compounding is a legal process in which US drug makers are allowed to sell medication that is not approved by authorities but offers alternatives for patients with specific needs.

Hims & Hers weight-loss drugs are compounded, whereas Novo Nordisk's are not.

The US Food and Drug Administration (FDA) said on Friday it was taking steps to restrict the compounding of weight-loss drugs to "safeguard consumers from drugs for which the FDA cannot verify quality, safety, or efficacy".

Novo Nordisk referenced the FDA's concerns in its lawsuit announcement on Monday, saying Hims & Hers' compounded drugs "may contain dangerous impurities or incorrect amounts of active ingredients, which can result in life-threatening immune responses".

"This is Novo's way of fighting back," said Fulford.

The Danish firm has had a rocky few months. It announced thousands of job cuts in September and last week warned investors about a slump in profits due to expiring patents.

Novo Nordisk has also come under fire from the FDA.

The agency sent Novo Nordisk a letter on Thursday saying a TV advert for its Wegovy pill "misleadingly implies benefits beyond physical weight loss, such as emotional relief, reduced psychological burden, hope, or direction for patients' lives".

Novo Nordisk said in response it "takes all regulatory feedback seriously and are in the process of responding to the FDA to address their concerns regarding the advertisement's presentation".