Perrigo Co. investors swallowed a bitter pill Monday as Chief Executive Joseph Papa was tapped to leadValeant Pharmaceuticals International Inc., leaving behind a struggling business months after he rallied the stockholders to fend off what now looks a favorable takeover offer.

Shares of the company tumbled 18% in Monday trading to $99.40 as the maker of store-brand cold and allergy medicines slashed its guidance and announced Mr. Papa’s exit.

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