Photostability of mecobalamin in solid dosage forms was investigated at dispensing level. Commercial products of one tablet and five capsules were used in packaged and bared conditions. The sample products were exposed to light for 20 days at 1500 lux. Assay was by high-performance liquid chromatography. The amount of mecobalamin in tablet and capsules decreased with lapse of photoirradiation time. Significant difference in the rate of photodegradation was observed between the capsule dosage forms, possibly due to light-transmission properties of capsule shells or plastic blister films. Mecobalamin in one tablet and two capsules was stable in both cases of packaged and bared, and there was no significant difference in mecobalamin remaining between packaged and bared samples.