The battle for supremacy in one of the fastest-growing pharmaceutical markets intensified on Monday, with CVS Health Corp. saying it will make Gilead Sciences Inc.’s drugs Sovaldi and Harvoni the exclusive options for patients with hepatitis C.

A competing treatment made by AbbVie Inc., called Viekira Pak, will be excluded from CVS’s drug formulary of approved medications, except in cases when it is medically necessary, CVS said in a letter sent to employment-benefit consultants that was reviewed by The Wall Street Journal....