The U.S. Food and Drug Administration on Friday approved Danish drugmaker Novo Nordisk's diabetes drug Tresiba, two years after rejecting the long-acting form of insulin.

The regulator also approved the company's Ryzodeg, an injection that combines Tresiba and a fast-acting manmade form of insulin.

The FDA approved the drugs with a warning that they should not be used in patients who have high levels of the chemical ketone.

Novo Nordisk, the world's largest diabetes drugs maker, separately said it expects to launch Tresiba in the United States during the first quarter of 2016.

Analysts were confident that the FDA would approve Tresiba, considered as a future blockbuster for the company.

The FDA had declined approval in 2013, asking for more data on risks of heart problems from using the drugs.

Tresiba is already being sold in 30 countries, and analysts expect annual sales of $2.4 billion by 2020, according to Thomson Reuters Cortellis.

Novo Nordisk earlier on Friday said its new experimental once-weekly diabetes drug, semaglutide, was successful in a late-stage study.

U.S.-listed shares of Novo Nordisk were little changed at $54.80 on Friday afternoon trading.