Gin and tonics, absinthe and Italian cocktails made with Campari and Pimm's liqueur all contain the bitter compound quinine, which comes from the bark of the cinchona tree of South America and was once the main defense against malaria. "Herb liqueurs all started this way," said Amy Stewart, a specialist in horticulture. "Apothecaries would soak the herbs and wood in alcohol to extract out the active ingredients and preserve them. Then you add a little bit of sugar to make it taste better, and you have a liqueur."