Mood drugs were chemical-releasing patches used by the New Humans of New Earth from sometime after 5,000,000,023 until 5,000,000,053, typically in the New New York Undercity. They generally gave emotions (such as Happy) although others affected the mind in other ways such as Sleep and Forget. In the Undercity they were mainly sold by the pharmacists. All in all, they were generally harmless.
However, one mood drug, Bliss, only found in the upper levels of New New York, was extremely addictive, and an airborne virus mutated inside, which spread and killed the entire population of the upper levels save the Face of Boe in his life-support tank, and Novice Hame, whom Boe surrounded with his smoke. (DW: Gridlock)