From Wikipedia:

[Nitrous oxide] was used on 30 September 1846 for painless tooth extraction upon patient Eben Frost by American dentist William Thomas Green Morton. Horace Wells Connecticut, a travelling dentist, had demonstrated it the previous year 1845, at Massachusetts General Hospital. Wells made a mistake, in choosing a particularly sturdy male volunteer, and the patient suffered considerable pain. This lost the colourful Wells any support. Later the patient told Wells he screamed in shock and not in pain.

From Trivia Library:

In December of 1846, Wells printed the results of his studies in anesthesia, but his ostracism by the Boston medical profession had left him an emotionally crippled man. He continued to experiment—on himself—with a variety of gases, including nitrous oxide, ether, and chloroform. These vapor inhalations strongly affected him emotionally, and he deteriorated mentally. In New York City in 1848, he sniffed chloroform, went berserk, and threw acid on the clothes of a prostitute. Jailed for this offense, Wells committed suicide by slashing an artery in his leg.