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Background  Because variable results of capsaicin challenges may be due to the incomplete solubility of capsaicin, we sought to determine if the use of Tween-80 in solutions of capsaicin improves actual concentrations of freshly prepared and stored solutions.    Methods  Capsaicin solutions ranging from 0.5–128 μM were mixed with and without Tween-80. Samples of various concentrations were then stored under 4 environmental conditions: 4°C, protected from light; room temperature, protected from light; room temperature, exposed to light; -20°C. All samples were analyzed initially, and at 2 and 4 months.    Results  While freshly prepared solutions with Tween-80 had consistently higher concentrations than those prepared without Tween-80 (83% vs. 69%), Tween-80 does not facilitate complete solubility. For solutions stored at 4°C and protected from light, there was a significant decrease after 2 months in low concentration solutions of both the Tween-80 and non-Tween-80 solutions. Both Tween-80 and non-Tween-80 containing solutions significantly decreased in concentration after 2 months when stored at room temperature and protected from light, room temperature and exposed to light, and -20°C. Concentrations of solutions made of 4 μM or higher are stable when stored at 4°C and protected from light for 4 months.    Conclusion  While the inherent difficulty of forcing capsaicin into solution cannot be eliminated, it can be improved with Tween-80. However, the addition of Tween-80 does not prevent the breakdown of stored capsaicin solutions. We recommend preparing and storing capsaicin solutions according to the methods and results of this study.

The effects of Tween-80 on the integrity of solutions of capsaicin: useful information for performing tussigenic challenges