Dimethyl sulfoxide (DMSO) is a clear, colorless and odorless organic solvent which is fully miscible with water. It stands out from many other solvents by being so harmless that it can be ingested in limited amounts (it is about half as toxic as ethanol) and can even be given intravenously. Its lack of harmful cellular effects is underlined by the fact that it is used to freeze down oocytes for later in-vitro fertilization. While melatonin is almost insoluble in water, it is readily soluble in DMSO, which is also used as a solvent for a variety of other sparingly soluble drugs. DMSO penetrates easily through cell membranes and through the skin, and has the property of carrying the dissolved drugs with it so that they achieve a higher intracellular concentration and thereby exert a more powerful effect.

In RepoCeuticals’ newly developed melatonin preparations for local administration, DMSO lies on the borderline between being an adjuvant and acting as an active ingredient with a synergic or additive effect. DMSO is itself a scavenger of harmful free radicals, soothes inflammation and also acts locally as a painkiller. The last two properties underlie the FDA’s approval of the agent, in a 50% mixture with water, as a bladder instillation for the symptomatic treatment of interstitial cystitis or painful bladder syndrome.