The degradation rates of hydrocortisone (HC) in oral liquid dosage forms (OLDs) prepared from HC tablets (Cortril^® tablets 10 mg, C-tabs) were investigated to achieve the safe use of OLDs. Ten milligrams of HC was dissolved in 40 mL of purified water (water) or tap water. The solution using water was called HC-water, and that using tap water was called HC-tap. To prepare OLDs, a C-tab was added to 40 mL of water (OLD-water) or tap water (OLD-tap). Stability tests on HC in HC-water, HC-tap, OLD-water, and OLD-tap were performed under the following conditions: at 40°C for 14 days, at 60°C for 7 days, and at 70°C for 3 days. HC was stable in HC-water. The residual percentages of HC in HC-water at 70°C was 98.8±1.39% (mean±SD, n=3). Apparent first-order rate constant (k) values (×10^-3 h^-1) for the degradation of HC in HC-tap, OLD-water, and OLD-tap at 70°C were 12.6±4.34 (mean±SD, n=3), 8.53±0.862, and 8.19±4.25, respectively. In Arrhenius plots, activation energies for the degradation of HC in HC-tap, OLD-water, and OLD-tap were 93.9, 74.9, and 114.9 kJ/mol, respectively. The results obtained on HC-water and HC-tap revealed that tap water had unknown substances that accelerated the degradation reaction of HC. The degradation of HC in OLDs was attributed to additives in the tablets. Unknown substances in tap water and reasons for the difference in degradation rates between OLDs remain unclear; therefore, further investigations are warranted.