Orally administered colonic purgative formulations and methods of its use for effecting partial or complete purgation of the colon in mammals, the formulations consisting of non-aqueous admixtures of a purgative salt selected from the group consisting of Mg.sub.3 (PO.sub.4).sub.2, MgHPO.sub.4, Mg(H.sub.2 PO.sub.4).sub.2, MgSO.sub.4, MgCl.sub.2, Na.sub.2 SO.sub.4, sodium tartrate, potassium tartrate, magnesium tartrate and mixtures, thereof, administered in tablet or capsule form in purgative effective concentrations. Preferred embodiments make use of at least one or more magnesium phosphate salts, more preferably dibasic magnesium phosphate; other preferred embodiments include the addition of binders, dispersants and buffers which do not adversely affect osmolality or effectiveness of the purgative formulations.