L-Arginine does not typically have known side effects when taken naturally. One presumed side effect to be cautious against was discovered during a clinical trial. An L-Arginine supplement was given to some patients following a heart attack, and in the end it did not improve vascular tone or the ability of the patients' hearts to pump. On the other hand, more patients taking L-Arginine died than did those taking only a placebo. The study was halted, and a recommendation against the use of the supplement by heart attack victims was prescribed.