Titanium dioxide (TiO2) has been widely used as a pharmaceutical excipient and is also known to be a strong photocatalyst. An investigation into the relationship between the photocatalytic activity of TiO2 and the photostability of famotidine, which is known as an H2-blocker, is presented. The photocatalytic activity of the anatase form of TiO2, as measured by the four-probe method, is approximately 1.5 times higher than that of the rutile form. Discoloration of famotidine in a binary system containing TiO2 depends significantly on both the wavelength of the irradiating light and the crystal form of the TiO2, with the degree of discoloration of anatase higher than that of the rutile form. Discoloration of famotidine also depends on relative humidity. The relationship between discoloration rate constant and water vapor pressure is linear. These results demonstrate that famotidine is easily discolored by the photocatalytic activity of TiO2 and suggest that the solid-state photocatalytic activity of TiO2 is strongly affected by relative humidity. Copyright 2004 Wiley-Liss, Inc. and the American Pharmacists Association