FRANKFURT, Feb 14 (Reuters) - Eli Lilly's Mounjaro drug is being launched in Britain this week, two pharmacy companies said on Wednesday, making the UK the fourth European country to introduce the highly-anticipated obesity drug.

British pharmacy and beauty chain Superdrug said on Wednesday that eligible private patients could seek prescriptions for Mounjaro, also approved for diabetes, through its Online Doctor service from Thursday.

The retail chain added, however, it would prioritise its stock of Mounjaro for National Health Service patients, citing likely strong demand as indicated by the popularity of Novo Nordisk's rival weight-loss drug Wegovy, which was introduced in the UK last year.

Britain's Simple Online Pharmacy told Reuters it started filling Mounjaro prescriptions and sending out packs on Tuesday.

"All pharmacies in the UK are able to order Mounjaro which is then available to both NHS and private patients," a spokesperson at Simple said.

Lilly declined to comment. The U.S. drugmaker said on Jan. 25 that it won regulatory clearance for pre-filling Mounjaro into an injection pen, allowing it to launch the drug in Britain within weeks.

It said at the time that Britain would be the first major market to be supplied with the drug in the pen that is good for four weekly subcutaneous shots for added convenience, branded as Kwikpen.

Other European markets -- Germany, Switzerland and Poland -- have seen Mounjaro launched before Britain. In those countries, the drug has for now been packaged in vials and patients need to draw the medication into syringes before injection.

Rival Novo, ahead in the race to serve a weight-loss market that some analysts expect to reach $100 billion in annual revenues, has been scrambling to boost production of Wegovy and of its lower-dose version Ozempic for diabetes, amid overwhelming demand. ($1 = 0.7964 pounds) (Reporting by Ludwig Burger Editing by Keith Weir)