Aspirin, which at the time was marketed as a "wonder drug," appears to have been deadly for people who mistakenly took gobs of tablets. Instead of ridding them of the flu, the aspirin led to Reye's syndrome. Dr. Karen M. Starko came up with the finding in a paper. From the NYT:

In February 1917, Bayer lost its American patent on aspirin, opening a lucrative drug market to many manufacturers. Bayer fought back with copious advertising, celebrating the brand's purity just as the epidemic was reaching its peak. Aspirin packages were produced containing no warnings about toxicity and few instructions about use. In the fall of 1918, facing a widespread deadly disease with no known cure, the surgeon general and the United States Navy recommended aspirin as a symptomatic treatment, and the military bought large quantities of the drug.

Don't blame it all on Bayer. At the time, the Journal of the American Medical Association suggested a dose of 1,000 milligrams every three hours, the equivalent of almost 25 standard 325-milligram aspirin tablets in 24 hours. That's roughly twice the daily dosage recommended today.