The Danish pharmaceutical giant that makes Wegovy, the world’s most popular weight loss drug, has become Europe’s most valuable firm, dethroning luxury conglomerate LVMH. The success of the treatment, which tricks people into thinking they are complete so that they eat less, has captivated celebrities, including actor Leonardo DiCaprio and Elon Musk, and has led to a glut in demand for the jabs. However, the drug’s popularity has also caused problems for the company. It has struggled to keep up with demand and has limited supplies to NHS customers because of a shortage of the injection pen that holds the drug semaglutide.

As a result, many high street pharmacies have been unable to sell Wegovy, and some big UK insurers have refused to cover it as part of a private medical insurance policy. Novo Nordisk has said it will continue to restrict supplies of Wegovy and its other GLP-1 diabetes and obesity treatments as it works to ramp up production.

Contract drug manufacturers seeking to tap into the booming market for weight-loss drugs are investing billions of dollars to expand or build factories that fill the injection pens used to administer treatments like Novo Nordisk’s (NOVOb.CO) Wegovy. The pens reduce the number of steps to use a drug and can be disposed of after each injection, reducing the risk of contamination and needlestick injuries. They are also designed to work well with increasingly popular safety devices and auto-injection systems.

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Interviews with a dozen company executives, analysts, and investors showed pharmaceutical services companies jostling to secure more specialist work filling syringes in the pens. Companies like Lonza (LON.DE), Catalent (CATT.N), and Fujifilm Diosynth Biotechnologies, a unit of the Japanese parent company, are all building capacity in this field. The work is known as “fill-finish.”

Fill-finish plants produce prefilled syringes in a wide range of injectable medicines, from life-saving vaccines to experimental cancer treatments. Using a cleanroom environment, they assemble drug-filled syringes and connect them to the devices needed to administer the medicine. This is done as a final step after pharmaceutical manufacturers complete the complex process of producing a drug from scratch and testing it to ensure quality and efficacy.

The prefilled syringes and parental drug manufacturing market has increased in recent years. 2015, it was worth $4.3 billion and is expected to reach nearly $6 billion by 2020. According to research firm Global Market Insights, this growth is driven by chronic disease and specialty drugs that require injection and a growing global aging population.

One significant change in the syringe and device industry has been moving away from glass for prefilled syringes, replaced by plastic and advanced technology that enables real-time monitoring of conditions inside the packaging to reduce contamination risks and ensure accuracy. The Internet of Things technology being used in this industry means that pharmaceutical firms can audit or supervise their contract manufacturer’s production more quickly than in the past.