Insulet stock was headed toward its worst day in 17 years on Wednesday. Although the medical device maker's second-quarter earnings beat expectations, its core business segment appears to be under pressure.

At first glance, quarterly numbers were solid. Second-quarter revenue rose nearly 23% in constant currency to $801.7 million, beating analysts' calls for $787.4 million. Adjusted earnings of $1.66 a share handily surpassed Wall Street's forecast of $1.45.

But shares sank 20% to $133.18 after the report on Wednesday, heading for the largest same-day drop since March 2009, according to Dow Jones Market Data. Insulet's losses made it the biggest laggard in the S&P 500.

The medical technology company generates nearly all of its revenue from one product line: Omnipod, an insulin pump system used to deliver continuous insulin for people with diabetes. Revenue tied to the product rose 24% to $795.9 million in the latest quarter.

Another business division didn't fare as well. Insulet's drug delivery unit, its specialized business-to-business arm, posted $5.8 million in revenue, down 78% from the previous year.

However, the real culprit behind the stock's decline was Insulet's dramatic cut to its Omnipod revenue guidance, which weighed down the rest of its outlook.

The company expects overall annual revenue to grow between 20% and 22%, down from a previous range of 21% to 23%. Within that, Omnipod sales are expected to grow between 21% and 23% for the full year, down from prior expectations of 22% to 24%.

That might seem like a slim cut, but RBC Capital Market's Kendall Au described it as "meaningful." The analyst added that third-quarter guidance for Omnipod revenue raises questions around the durability of Insulet's growth strategy. (Insulet forecast Omnipod revenue growth in the range of 18% to 20% for the third quarter.)

Citi Research analyst Joanne Wuensch was similarly cautious. Insulet "has become a well debated stock, which will likely continue today," she wrote. "We anticipate concerns over the lowered guide and concerns amidst increasing competition."