Characterization of Tetracycline Hydrochloride Compounded in a Miracle Mouthwash Formulation

Miracle mouthwash (MMW) is a commonly prescribed oral formulation compounded with varying active ingredients, depending on purpose of treatment. Due to patient-to-patient customization, the solubility, stability, and solid-state characteristics of the active ingredients may not be known after compounding. This study found that the common antibiotic, tetracycline hydrochloride (HCl), compounded in MMW formulations that contained dexamethasone elixir and diphenhydramine, underwent significant physical-chemical changes. Simulated patient conditions demonstrated appreciable fluctuations from the target content of 50 mg tetracycline HCl per teaspoon over 15 days. The lowest tetracycline content sampled was 32.5 mg, while the highest content sampled was 53.0 mg. Although tetracycline HCl went into solution after compounding, tetracycline did not remain in solution. In fact, the amount of tetracycline in solution declined exponentially, with over two-thirds of tetracycline precipitating out within the first day of compounding and 14% remaining in solution after 15 days. Crystals that formed within the MMW formulation were analyzed using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and powder X-ray diffraction (PXRD), which confirmed a solvent-mediated phase transformation of tetracycline HCl to tetracycline hexahydrate. For tetracycline in solution, pH had a significant effect on chemical degradation. Therefore, tetracycline HCl compounded in MMW formulations can have significant physical-chemical stability changes, possibly impacting patient dosing.