Is it obligatory to obey a mother who takes excessive amounts of medication in a way that may harm her, and invokes illness to achieve her desires, or is it permissible to stop bringing expensive medication that she does not need and that may harm her, especially given the urgent need for its cost?

It is not permissible to obey parents in a matter prohibited by Sharia, such as bringing harmful or unnecessary medicine, as this constitutes aiding in self-harm, which is forbidden by the Quran and Sunnah, and is also an act of extravagance and waste. You should try to convince your mother with wisdom and seek help from doctors.