Context and causation

Long-term:

Yes, there were cases before the Tylenol murders, including one where a doctor handed out poisoned medicine.

"But to realize that six human beings, all of them my patients, one of them my best friend, are dead because they took medicine that I prescribed for them innocently, and to realize that that medicine which I had used for years in such cases suddenly had become a deadly poison in its newest and most modern form, as recommended by a great and reputable pharmaceutical firm in Tennessee: well, that realization has given me such days and nights of mental and spiritual agony as I did not believe a human being could undergo and survive. I have known hours when death for me would be a welcome relief from this agony." (Letter by Dr. A.S. Calhoun, October 22, 1937)

Before the Tylenol murders there was a case where a doctor unknowingly handed out poisoned medicine to six patients. Similarly to the Tylenol murders where no one was specifically targeted

Short-Term:

Lewis (the guy pretending to be the person who poisoned the Tylenol) admitted to writing a letter to embarrass his wife's former employee.

“Lewis sent Johnson & Johnson”... “a letter saying he would ‘stop the killing’ if the company paid him $1 million. Lewis was charged and convicted of extortion in 1983, and was sentenced to 12 years in federal prison. During his trial, Lewis’ attorney conceded that his client wrote the letter, but said Lewis only did it to embarrass his wife's former employer,” (Sennott).

He wanted to frame his wife's former employer on something she didn't do to embarrass her and no one knows who did it to this day.