As of 2015, there is no credible medical evidence that yellow mustard can help treat leg cramps. Mayo Clinic suggests treating leg cramps with stretching exercises, massages, and warm showers or baths.
Some websites, such as Health911, recommend ingesting a spoonful of mustard to treat leg cramps. These websites state that cramps may be caused by a deficiency in acetylcholine, a neurotransmitter involved in muscle movement. Mustard contains acetic acid, which is necessary for the body's production of acetylcholine.
However, neither WebMD nor Mayo Clinic list any kind of mustard as a treatment for leg cramps. Stretching, staying hydrated and eating foods high in magnesium and calcium may prevent leg cramps, according to WebMD.