Dexcom shares sank more than 40% on Friday and headed for their worst day ever after the diabetes management company reported disappointing revenue for the second quarter and offered weak guidance. The stock fell $45.38 to $62.47 as of early afternoon, wiping out about $18 billion in market cap. Prior to Friday, the biggest drop came in September 2017, when the shares plunged 33% in a day. Dexcom held its stock market debut in 2005. With Friday's selloff, Dexcom shares are down almost 50% for the year, while the S&P 500 is up 15%.

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Persons: Dexcom, Kevin Sayer, we've, Sayer, JPMorgan's Robbie Marcus, Marcus, Jereme Sylvain, Sylvain, William Blair, Leerink, Stelo, Friday's Organizations: JPMorgan, U.S . Food, Drug Administration