This mnemonic, with some variations, has been used for a long time as a reminder for the symptoms of the anti-cholinergic toxidrome and in general to describe antimuscarinic side effects of medications. 

When was it actually first used in the medical/pharmaceutical literature?

The oldest I can find on PubMed is the following article:

Feinberg M. The Problems of Anticholinergic Adverse Effects in Older Patients. Drugs Aging. 1993;3(4):335-348. doi:10.2165/00002512-199303040-00004.

There must be older articles or textbooks that have used the expression.

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