Chester was a poor choice to endorse the professor's elixir. The professor claimed that his elixir would cure anything that ails you. But it evidently didn't grow Chester a new leg.
Everyone was shocked when Doc revealed his finding of opium in the professor's elixir. Matt promised to run the professor out of town. But opium was a common ingredient in 19th century patent medicines and people thought nothing of it. In fact opium could be purchased over the counter by anyone without a prescription. The first significant federal regulation of opium was the Harrison Narcotics Tax Act of 1914.