A process for preparing a calcium channel antagonist effective for dual purpose of inhibiting secretion of parathyroid hormones and of promoting absorption and utilization of calcium, said calcium channel antagonist comprising an acid extract of a calciferous material of plant origin, which comprises:
- baking a calciferous material of plant origin under reduced pressure or vacuum at a temperature of 300-1000° C., and
- soaking the baked material in sulfuric acid and a sulfuric acid extract is obtained from the soaked baked material.