How can one perform ghusl for prayer when a headache intensifies after performing ghusl for janabah?

Water must reach the skin and the roots of the hair when washing the head after major ritual impurity (janabah (definition)). However, if the questioner is harmed by washing her head, either through personal experience or a trusted doctor's advice, then it is permissible for her to wipe it instead of washing it, because Allah Almighty "has not placed upon you any hardship in religion," and removing harm is one of the principles of Islamic jurisprudence.