The basic principle with regard to interest is that it should be got rid of in ways that do not bring any benefit to the one who is disposing of it. So it should be spent on good causes and in the public interests of the Muslims.
It is not permissible to give it to your parents for the purpose of maintenance, because if they are poor, you are obliged to spend on their maintenance, so giving the interest to them would be saving your wealth and would bring benefit to you.
But it is permissible to give it to them to pay off their debts, because the son is not obliged to pay the debts of his parents.
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