The present invention pertains to stabilization of a certain class of benzothia(dia)zepine derivative, in a solid preparation containing the derivative. According to the present invention, either the solid preparation containing the benzothia(dia)zepine derivative does not incorporate a combination of polyethylene glycol and polyvinyl alcohol; or, in the case that the solid preparation does incorporate a combination of polyethylene glycol and polyvinyl alcohol, the derivative and the combination of polyethylene glycol and polyvinyl alcohol are isolated.