The group said its 2019 "average prices after rebates are expected to be lower" compared to 2018 in the United States, where it generates almost half of its turnover, due to a change in legislation which reduces prices on prescription drugs.
The share price fell nearly 4.8 percent around 0930 GMT on the Copenhagen stock exchange in a market down by 0.83 percent.
For the second quarter, the group posted a net profit of 10.34 billion kroner (more than 1.3 billion euros, $1.5 billion), up by four percent year-on-year.
Sales, which fell by 4.3 percent to 27.4 billion kroner, were weighed down by a four percent decline in diabetes treatments by Novo Nordisk's flagship brands such as Tresiba and Victoza, which represent 84 percent of the company's total sales.
Sales of haemophilia treatments and growth hormones also fell 10 and 4 percent, respectively.
The group said "sales growth is expected to be partly countered by intensifying global competition both within diabetes care and biopharmaceuticals" and "continued pricing pressure within diabetes care, especially in the USA".
Analysts surveyed by the Stockholm-based independent consensus estimate service SME Direkt had expected sales of 27.7 billion kroner.
The depreciation of the US dollar compared to the Danish krone also had a negative effect on sales.
Forecasts for 2018 have been left unchanged, with local sales up 3 to 5 percent and a rise in operating profit by 2 to 5 percent.
Novo Nordisk controls nearly half of the global market for insulin. Its haemophilia and hormonal treatments represent a fifth of its turnover.