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The chief historical examples are rhubarb, senna, santonin and logwood for urine; iron preparations, bismuth preparations and charcoal for darkening the faeces. There are two different processes: a substance absorbed and subsequently excreted can colour a secretion, while material remaining in the intestinal lumen can colour the stools without demonstrating systemic drug absorption.
- Rhubarb colours urine yellow to yellow-brown; the colour becomes redder in an alkaline solution. The contemporary account also reports coloration of sweat and breast milk. This is a historical observation about passage of colouring matter, not a claim that every constituent passes unchanged into every secretion. [ref]The Essentials of Materia Medica and Therapeutics by Alfred Baring Garrod (1865). Labiatae, continued, Rheum: colouring constituent, alkaline solutions, and coloration of urine, perspiration and milk
- Senna can give urine a yellow-brown or red-brown colour. The effect is still recognised in current medicinal information. [ref]Senna by the NHS. Side effects, coloration of urine
- Santonin, used historically as an anthelmintic, can make urine conspicuously yellow. This must be distinguished from the drug's separate effect on colour vision: a yellow-looking visual field is not itself excretion of a yellow pigment. [ref]A Manual of Practical Therapeutics by Edward John Waring (1865). §2416, physiological effects and yellow urine
- Logwood can give urine a pink or reddish colour, particularly when it is an alkaline solution; the historical description notes that addition of Ammonia brings out the colour in a previously acid specimen. [ref]The Essentials of Materia Medica and Therapeutics by Alfred Baring Garrod (1865). Cisalpinae, Haematoxylum, urine coloration
- Iron preparations can darken faeces; bismuth preparations also cause dark or black stools. In these examples the result need not be a pre-existing vegetable dye simply passing unchanged through the body. Current information confirms black stools with oral iron and with the modern bismuth medicine bismuth subsalicylate; the latter is corroboration of the colour effect, not an identification of that modern formulation with the 1864 bismuth articles. [ref]Bleeding from the bottom by the NHS. Dark stools and iron tablets[ref]Pepto-Bismol by the NHS. Side effects: black stools and black tongue
- Charcoal supplies a black material directly to the bowel contents. Its appearance in the stools does not require its absorption and subsequent renal excretion.
Colour alone does not identify either the drug or a disease. A medicinal pigment can resemble blood or a bile pigment; pH, timing after administration and appropriate chemical or microscopic examination distinguish possibilities. Conversely, a history of taking a colouring medicine does not prove that an observed specimen contains no blood. These examples describe pharmacological and historical observations, not results from an unseen specimen.