= Solution
\b[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. <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]{ref}
* <Senna> can give <urine> a yellow-brown or red-brown colour. The effect is still recognised in current medicinal information. <Senna by the NHS>[Side effects, coloration of urine]{ref}
* <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. <A Manual of Practical Therapeutics by Edward John Waring (1865)>[§2416, physiological effects and yellow urine]{ref}
* <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. <The Essentials of Materia Medica and Therapeutics by Alfred Baring Garrod (1865)>[Cisalpinae, Haematoxylum, urine coloration]{ref}
* <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. <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]{ref}
* <Charcoal> supplies a black material directly to the bowel contents. Its appearance in the stools does not require its <absorption> and subsequent <renal excretion>.
\b[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.
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