You can't just insert a pill and expect it to absorb, because the rectum produces only minimal amount of mucous; it lacks the ability to break down pills mechanically, or dissolve them.
The actual structure of the drug molecules themselves are pretty much the same - it's just that rather than a solid base like cellulose, talc, lactose, or starch, one uses refined waxes or glycerin-gelatine mixtures.
Martindale 1941 - Suppositoria
Suppositories are medicated masses intended for anal administration. They are usually conical at one end, the other end being flat so that the the suppository is more easily retained by the sphincter muscle after insertion. It is customary to use moulds which hold 1 gram (15 grains) or 2 grams (30 grains) of oil of theobroma. (ed. note, in modern terms - cocoa butter) Unless otherwise specified, the 15-gr size is supplied.
The bases employed for suppositories are oil of theobroma, glycerin suppository mass and occasionally freshly prepared soap, as in glycerin soap suppositories. It is essential that the melting point of any suppository shall be between 30 and 35 degrees Celsius. (ed. for the Americans, 37 degrees +/- 0.5 deg = average internal body temperature) Oil of theobroma is usually employed unless the prescriber otherwise directs, and when the suppositories are prepared by melting and molding, care should be taken not to overheat this base since, as with many other substances, overheating will cause a lowering of the solidifying-point and subsequent difficulty in setting. (ed. in other words, cocoa butter tends to super-cool) Because of this it is preferable to use powdered or shredded oil of theobroma. Certain medicament such as phenol, chloral hydrate, and resorcinol (ed. please don't try this at home!) cause an appreciable lowering of the melting-point of oil of theobroma when warmed with it. In preparing such suppositories, the melting point may be brought back to normal by incorporating a little white wax. (ed. paraffin wax) This addition may be avoided, except in the case of suppositories containing volatile oils, by using the minimum amount of heat.
