Novo Nordisk will have to wait a fair bit longer for US approval for its ultra long-acting insulin degludec and insulin combination analogue degludec/aspart after regulators put back their review on the drugs.

The US Food and Drug Administration has extended the regulatory review period for the degludec and the aforementioned combo for type 1 and 2 diabetes by three months. The agency has asked for "further data clarification and analyses" and Novo has already submitted "a substantial amount" of that additional information.

However, "due to the size and timing of these submissions", the FDA considers them as major amendments to the New Drug Applications for the therapies. Novo also stressed that the agency has not requested additional clinical trials and a decision on the drugs, also known as Tresiba and Ryzodeq, is now expected by October 29.

If approved, degludec will represent a major challenge to Sanofi's Lantus (insulin glargine), the world's biggest-selling insulin product. The two products have also been filed with the European Medicines Agency and regulators in Japan, Canada, Switzerland and elsewhere.

Investors have been spooked by news of the delay and Novo shares were down 3.2% at 9.35am (UK time) to 778 Danish kroner.