Medicine
Medicine was not very regulated in this decade. Absolutely anyone could sell pills that could promise anything.
Heroine is first introduced in 1898. People originally thought that it did not have addictive qualities, but once they found out otherwise they banned it from public use.
Aspirin was introduced into the medical practice in 1899. This was really the first safe pain killing drug that was available in this time period that they knew how to use.
Pharmacists frequently prescribed heroine, cocaine, morphine, and opium.