Martin A. Erickson III, RPh
Published Online: Friday, July 1, 2005

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-like concoction, and it measured less than 30 mL. Because of the addition of 30% lactic acid liquid, I cut the volume of flexible collodion accordingly.

A: This formulation presents an interesting weight/weight versus weight/volume problem that is the center of periodic debates in dispensing pharmacies. Due to space considerations, the discussion will be limited. Practical realities of dispensing lead to the irrelevance of this debate if the compounder does not work quickly; concentrations of the ingredients change erratically, as discussed below.

Lactic acid USP is 88% to 92% lactic acid in solution. When lactic acid is called for, unless otherwise specified, consider the lactic acid 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, in the situation you describe, evaporation of volatile ingredients (ether, particularly) resulted in the insolubility of the camphor, castor oil, and pyroxylin. Work quickly and in a closed container, if possible, when compounding this product.

Suggested procedure: Weigh the salicylic acid, and measure (weigh) the lactic acid. Then place the salicylic acid in a graduated cylinder (the smallest size available to accommodate the Rx volume and to minimize the "head space"), and add the lactic acid. Stir to mix. Finally, add the flexible collodion to volume (30 mL), cover, and agitate gently but thoroughly to combine the ingredients. Transfer the ingredients to the dispensing container.

Caution the patient to keep the product tightly closed. Even with this precaution, opening the container periodically will allow for the evaporation of volatile ingredients, leading to a solid product within a short time (usually about 30 days). When applications are required, the patient should open the container, withdraw the needed amount 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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