CVS Health (CVS) stock tumbled 10% Friday morning after the pharmacy and healthcare giant announced the replacement of Chief Executive Officer (CEO) Karen Lynch with David Joyner, a longtime company veteran who most recently ran Caremark, its pharmacy benefit manager (PBM).

Joyner is taking the top job effective immediately, CVS said, as the company has weathered a difficult year of sales at its namesake pharmacies and problems with its bets on Medicare coverage that have led it to lower its full-year outlook several times already.

CVS Board Says Was 'Right Time To Make a Change'

Newly appointed executive chairman Roger Farah said the board believes it was "the right time to make a change," and said Joyner's decades of experience with CVS "can help us more directly address the challenges our industry faces" and make necessary changes.

In addition to the CEO swap, CVS also provided preliminary guidance for third-quarter profit, projecting earnings per share (EPS) between 3 cents and 8 cents, and adjusted EPS of $1.05 to $1.10. Analysts are expecting EPS of $1.27, and adjusted EPS of $1.69, according to consensus estimates compiled by Visible Alpha.

The company is set to report third-quarter earnings Nov. 6, and said the lackluster projections are connected to costs related to its Medicare operations and other one-time expenses like store closures.

The Wall Street Journal first reported the news that the CEO change was imminent, sending CVS stock lower Friday morning. After the announcement, shares were down more than 11% to $56.30, nearly 30% below where they started the year.