Doctors have prescribed quinine to treat leg cramps, but the amount of quinine in tonic water is too small to pose much of a benefit, as Harvard Health indicates. There are about 15 milligrams of quinine in 8 ounces of tonic water.
Quinine is no longer deemed to be safe or effective for treating or preventing leg cramps, according to the U.S. Food and Drug Administration. Side effects from the use of quinine are risky and potentially deadly, such as severe interactions with other medications, reports WebMD. From 1969 to 2006, there were over 600 reports of serious adverse reactions to quinine, including 93 deaths.
The small amount of quinine in tonic water is unlikely to cause the adverse side effects noted in prescription-strength quinine, according to Consumer Reports. Doctors have prescribed quinine for treating leg cramps since the 1940s, but the FDA has been issuing warnings about the risks of quinine and its minimal effectiveness for leg cramps since 1994. However, an audit from 2008 to 2011 determined that about 92 percent of quinine use was still related to treating muscle pain and leg cramps, states the FDA.