June 20 (Reuters) - U.S. drugmaker Eli Lilly on Thursday said it was suing six more U.S. medical spas, wellness centers and other entities for selling products claiming to contain tirzepatide, the active ingredient in its popular diabetes drug Mounjaro.

Tirzepatide was approved late last year as Zepbound for weight loss, and Lilly is the only company that has the approval of the U.S. Food and Drug Administration to sell the drug.

In its six separate lawsuits filed in the courts of Ohio, Texas, Hawaii and the District of Columbia, Lilly has alleged that some of these entities were falsely claiming that their compounded versions of tirzepatide are approved by the FDA.

In an open letter, Lilly warned that unsafe or untested compounded tirzepatide puts patients at risk and that it stands against the use of its treatments for cosmetic weight loss.

Demand has been outpacing supply for GLP-1 drugs, which include Novo Nordisk's Wegovy and Lilly's Zepbound and Mounjaro.

Some analysts have forecast that the market for these new weight-loss drugs, which work by suppressing appetite and promoting a feeling of fullness, could reach $150 billion by the early 2030s.

Lilly and Danish rival Novo have sued several entities to stop them from selling products claiming to contain the active ingredients - tirzepatide and semaglutide - used in their respective popular weight-loss drugs.

Last year, Lilly took similar actions against 10 spas, wellness centers and compounding pharmacies. (Reporting by Bhanvi Satija in Bengaluru; Editing by Anil D'Silva)