Emodin or Anthraquinone Cathartics

The pure substances would be too irritant, but their action is graded by their slow liberation and by the presence of colloid extractives. The most common of these substances are emodin and chrysophanic acid. Numerous isomers of these are possible (15 for emodin alone). The special character of the different drugs is probably due to differences in these isomers, in the stability of their glucosidal combination, and to the presence of other associated substances (tannin in rhubarb). 

Emodin is partly absorbed and is excreted into the urine and milk (laxative effects on sucklings). The drugs containing emodin color the urine yellowish-brown when it is acid, and reddish or violet when alkaline. It is advisable to warn the patient about this color change. 

Other drugs that change the color of the urine act as follows. Logwood (hematoxylon) does not color acid urine but produces a reddish or violet color in alkaline urine. Santonin imparts a yellowish color to acid urine, with a yellow foam if the urine is made alkaline, it imparts a very pronounced pink color. Picric acid gives reddish-brown color in both acid and alkaline urine. The various coal-tar products give a brownish-black color. Methylene blue imparts a green color. 

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