Dowdell, Govindaraj, and Jain (1992)
Dowdell, Thomas, Suresh Govindaraj, and Prem Jain. "The Tylenol Incident, Ensuing Regulation, and Stock Prices." Journal of Financial and Quantitative Analysis, June 1992.
This event study reviews daily stock returns for Johnson & Johnson and 28 other healthcare firms for 28 days (2 prior to the Tylenol incident, 25 after). During the first nine days, only Johnson & Johnson had abnormal returns (-29%, t=-3.5). In the following 19 days, regulations were imposed. Other drug stocks were then affected (-12%, t=-4.0).
More information on the 1982 "Tylenol Incident" is here.