Shares of Dexcom (DXCM) tumbled over 40% in extended trading Thursday after the company repor👍ted second-quarter earnings that missed estimates and slashed its full-year revenue guidance.

The maker of glucose monitors said it now expects full-year organic revenue growth of 11% to 13%, down from its projection in April of 17% to 21%.

Dexcom's revenue in the second quarter grew 15% year-over-year to $1 billion, roughly in line with analysts' estimates compiled by Visible Alpha. However, net income of $143.5 million or 35 cents per share missed expectations.

“While Dexcom advanced several key🦩 strategic initiatives in the second quarter, our execution did not meet our high standards,” CEO Kevin Sayer said in a release, adding “we are taking action to improve our execution and best position ourselves for continued long-term growth.”

Dexcom also announced a $750 million stock buyback program.

Shares of Dexcom were down 40.1% at $64.50 in extended trading as of 5:40 p.m. ET Thursday following the release.