My father (75) had a brain stroke 10 months ago and he has been on plavix (clopidogrel) all this time. For how long should he stay with it? Can he pass to aspirin?

Prevention. Would not recommend aspirin, as the data shows relatively mediocre results unless he would take one full asa, 325mg four times daily, which could cause gastritis and bleeding. Based on research data, the most successful protection against a subsequent stroke comes from Aggrenox twice daily, and if it were my dad, i would consider this first.