Given that one may not intentionally injure herself - see Rambam (Chovel U’mazik 5:1) and Shulchan Aruch (Choshen Mishpat 420:31) who rules that it is forbidden for a person to injure herself. Moreover, many authorities have opined that the prohibition is a Torah Law (see Rashba 1:647; Tumim 27:1).
May one intentionally cause herself to experience pain by refusing painkilling medication offered during a medical procedure?
see also Bava Kama 90b