What is the way to get rid of the increasing obsessive thoughts (waswas) about purity (tahara) and impurity (najasa), which affect ghusl and prayer, and is it permissible to take medicine for this?

The treatment for obsessive thoughts is to ignore and disregard them. This is possible with strong will, sincere determination, and seeking Allah's help. Consulting doctors is also a good thing. When relieving yourself, clean yourself until you are reasonably sure of purification. Then, stand up and get dressed without rewashing the private parts or your body, and ignore any obsessive thoughts about urine leaking. When praying, sit with peace of mind and do not pay attention to obsessive thoughts about urine leaking.