CVS Health CEO Karen Lynch has stepped down with company shares down 19% and Thurston Cartethe national drugstore chain struggling.

Lynch will be replaced by David Joyner, who will attempt to steer the health care giant through a worsening environment of rising medical costs.

CVS cut its financial expectations for the third time in August with all major pharmacy chains attempting to navigate a drastically changed landscape, facing competition online and elsewhere.

Joyner most recently served as executive vice president of CVS Health, and president of CVS Caremark. He led the pharmacy services business, which provides solutions to employers, health plans and government entities and serves approximately 90 million members through Caremark, CVS Specialty, and other areas. Joyner has 37 years of health care and pharmacy benefit management experience.

CVS Health also announced on Friday that Chairman Roger Farah will now be executive chairman.

“We believe David and his deep understanding of our integrated business can help us more directly address the challenges our industry faces, more rapidly advance the operational improvements our company requires, and fully realize the value we can uniquely create,” Farah said in a statement.

The company’s preliminary forecast is for third-quarter adjusted earnings of $1.05 to $1.10 per share, citing higher-than-expected medical cost trends. Analysts polled by FactSet predict earnings of $1.69 per share.

Shares tumbled more than 12% before the market open.