Is it legislated to say "O Healer, O Sufficer" before taking medicine to seek healing, and does it have a basis in the Sunnah?

This supplication was not mentioned in the Prophetic Sunnah. However, it is permissible for a sick person to supplicate to Allah Almighty at all times for the fulfillment of their need, using any supplication they wish, and mentioning Allah's Most Beautiful Names and Attributes that suit their request, without believing in any special merit for this particular supplication. Sheikh Abd al-Rahman al-Barrak stated that "there is no harm in that; because this is a supplication that has a present occasion, and it is not considered a form of dhikr (remembrance of Allah)."