Skin hyperpigmention (or skin darkening) is a less common side effect, which typically occurs close to where the solution is applied on the skin. This is due to an increase of melanin in the skin. The pigmentation is expected to increase as long as bimatoprost is administered, but has been reported to be reversible in most patients upon discontinuation of bimatoprost. Skin hyperpigmention (or skin darkening) is a less common side effect, which typically occurs close to where the solution is applied on the skin. This is due to an increase of melanin in the skin. The pigmentation is expected to increase as long as bimatoprost is administered, but has been reported to be reversible in most patients upon discontinuation of bimatoprost.