Belgium’s UCB is celebrating the news that US regulators have given the green light to Cimzia as a treatment for Crohn's disease, though the approval comes with a warning of potentially serious side effects.

The US Food and Drug Administration has approved Cimzia (certolizumab pegol) for adults with moderate to severe Crohn's disease who have not responded to conventional therapies. The approval, which was based on safety and efficacy data from clinical trials in more than 1,500 patients, is “a significant milestone” for UCB, said chief executive Roch Doliveux as the USA represents 70% of the world Crohn’s anti-tumour necrosis factor alpha market.

Julie Beitz, director of the Office of Drug Evaluation III for the FDA's Center for Drug Evaluation and Research, said that “this drug works to reduce the signs and symptoms of Crohn's, but it also carries risks that will require patients on it to be closely monitored". The agency noted that patients taking Cimzia are at increased risk for serious adverse effects, including “infections that can lead to hospitalisation or death”.

Because the UCB drug affects the immune system, it can lower the body's ability to fight infections, such as tuberculosis, said the FDA and seeing as Cimzia is a blocker of TNF, it may cause lymphomas and other malignancies. The agency added that although an increased risk of tumours was not seen in studies of Cimzia, “the modest size and relatively short duration of the controlled studies prevents any firm conclusion”.

As a result the FDA noted that post-marketing studies and clinical trials will be required to obtain long-term safety data. Nevertheless, the approval is a huge boost for UCB as analysts were concerned whether the treatment would ever get the nod.

Last year, the firm was asked by the FDA for additional information on data submitted in its Biologics License Application for the Crohn's disease indication (it is also under review for rheumatoid arthritis). UCB submitted data from a fresh trial which has done the trick in the USA but the firm’s attempts to get approval in Europe have not been so successful.

Last month, the European Medicines Agency’s Committee for Medicinal Products for Human Use upheld a negative opinion on Cimzia saying that certain issues had been resolved during the appeal process brought by the company, including fears over a possible increased risk of bleeding associated with the product’s use. However it said it remains concerned over the drug’s general safety and effectiveness profile.

Now Cimzia can start trying to take market share of Abbott Laboratories rival Crohn's disease drug Humira (adalimumab), which was approved by the FDA over a year ago. The latter carries similar warnings to Cimzia, which is a vitally important drug for UCB, which lost patent protection on its biggest earner – the allergy drug Zyrtec (cetirizine) – last year.