This section is from the book "A Text-Book Of Materia Medica And Pharmacy For Medical Students", by Velyien Ewart Henderson. Also available from Amazon: A Text-Book of Materia Medica and Pharmacy for Medical Students.
1. Resulting in precipitation of one of the ingredients in solution owing to its decreased solubility when its solvent is diluted by another liquid. The dilution of aqueous solutions of acacia, proteins, salts (if strong). and emulsions with alcohol. Some gums as well as starches and dextrins are similarly precipitated by alcohol. In some of the cases that will occur under this rule the precipitate is not an important constituent, for example, the Liquid Extract of Cascara Sagrada gives a precipitate with alcohol, the precipitate consists however of unimportant constituents and may be filtered off. The dilution of alcoholic solutions of resins, oleo-resins, oils, etc., by water. In some of these cases also the precipitate is unimportant, for example, Liquid Extract of Nux Vomica and water.
It must be distinctly understood that at times it is advisable or even necessary to order incompatibles in a prescription. Attention might be called to the fact that the Pharmacopoeia contains such formulae, for example the Lotio Hydryrgyri Nigra, and the Mistura Ferri Composita. Whenever the physician orders such a preparation he should warn the patient that the bottle will contain a deposit. It is only rarely that one should write such a prescription as will involve an uncorrected incompatibility. No prescription should ever be written which if dispensed would lead to the precipitation of any highly active ingredient, as in the cases of such a precipitate the patient might readily be poisoned by getting an over-dose of the potent precipitate in the last dose. The practitioner should make it the rule to send out preparations free from precipitate and of an attractive colour. In some cases the incompatibility may be overcome; for example the carbonates and the bicarbonates, the bromides and iodides of the alkaloids while less soluble in water than the usual salts, are comparatively soluble in alcohol, and hence the addition of alcohol will prevent the precipitation. In other cases it may be possible by increasing the viscosity of the mixture by the addition of acacia, tragacanth or syrup to prevent the formation of a precipitate or much more often the addition of one of these ingredients will so prevent the clotting of the precipitate that it may be safely dispensed with a "Shake the Bottle" label.
There are some substances such as, the salts of silver, phenacetin, phenazone, potassium iodide and calomel that react with so many other drugs that it is preferable to administer them alone, or in simple solutions with a flavouring reagent or in pills.