Q: A wart-removing compound was ordered as follows: 30%salicylic acid and 30% lactic acid in flexible collodion. Disp:25 to 30 mL.
When I mixed these ingredients together, I got a thick lotion-likeconcoction, and it measured less than 30 mL. Because of theaddition of 30% lactic acid liquid, I cut the volume of flexible collodionaccordingly.
A: This formulation presents an interesting weight/weight versusweight/volume problem that is the center of periodicdebates in dispensing pharmacies. Due to space considerations,the discussion will be limited. Practical realities of dispensing leadto the irrelevance of this debate if the compounder does not workquickly; concentrations of the ingredients change erratically, asdiscussed below.
Lactic acid USP is 88% to 92% lactic acid in solution. When lacticacid is called for, unless otherwise specified, consider the lacticacid USP as "100%"; do not try to "adjust"for the USP specification.Therefore, the lactic acid USP is 30% of the product.Flexible collodion contains ether (which is very volatile), alcohol(also volatile), castor oil, camphor, and pyroxylin. Most likely, inthe situation you describe, evaporation of volatile ingredients(ether, particularly) resulted in the insolubility of the camphor, castoroil, and pyroxylin. Work quickly and in a closed container, ifpossible, when compounding this product.
Suggested procedure: Weigh the salicylic acid, and measure(weigh) the lactic acid. Then place the salicylic acid in a graduatedcylinder (the smallest size available to accommodate the Rxvolume and to minimize the "head space"), and add the lacticacid. Stir to mix. Finally, add the flexible collodion to volume (30mL), cover, and agitate gently but thoroughly to combine theingredients. Transfer the ingredients to the dispensing container.
Caution the patient to keep the product tightly closed. Even withthis precaution, opening the container periodically will allow for theevaporation of volatile ingredients, leading to a solid productwithin a short time (usually about 30 days). When applications arerequired, the patient should open the container, withdraw the neededamount of material on an impervious applicator (usually plastic),and reclose the container tightly before applying the material.
Mr. Erickson is director of professional affairs at Gallipot Inc.
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