Duff beer was a beer produced on Simpson's Planet that could cure llhrei'sian on Romulans. Its alcoholic content was 11 Proof (5.5% abv) and was rather brown in coloration. (RIS Bouteina)
Vox'ula realized that Duff beer was a cure for the llhrei'sian induced by Krusty burgers, after drinking it. Also, Homer Simpson was fond of Duff beer and could tell between replicated and fresh Duff beer. ("What about some D'oh!")
Background[]
The real-world Duff beer is manufactured by the Daleside Brewery. The Duff beer as portrayed in RIS Bouteina bore a strong resemblance to the real drink, since it was rather dark.