This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

v. Third-person singular simple present indicative form of tranquillize.

"Those, however, over whose nerves it spreads only a delightful calm, whose feelings it tranquillizes, and in whom it produces an habitual state of reverie, are those who should be upon their guard lest the drug to which in suffering they owe so much should become in time the direst of curses."

tranquillized    tranquillizes    tranquillizing   

tranquillity    tranquillizer