ABSTRACT: 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 muM were mixed with and without Tween-80. Samples of various concentrations were then stored under 4 environmental conditions: 4 degrees C, protected from light; room temperature, protected from light; room temperature, exposed to light; -20 degrees 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 degrees 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 degrees C. Concentrations of solutions made of 4 muM or higher are stable when stored at 4 degrees 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.