What is the ruling on taking a monthly loan from the National Bank to meet needs and treat a chronic illness, given a small income and inherited wealth from which one lives off its returns?

If a bank is usurious (Riba-based), then depositing money in it or borrowing from it with a stipulated increase is impermissible. The interest gained from it is forbidden Riba, and it is not permissible to benefit from it. As for usurious borrowing, it is among the gravest sins and should not be undertaken except in cases of extreme necessity, such as when leaving a treatment would lead to destruction, and there is no other permissible way to cover the expenses.