by estrogen blocker im guessing you mean an AI? well to answer your question, can you? yes. should you? prolly not. the thing is with most otc steroids is that they dont aromatize. for example, mdrol does not turn into estrogen. however, if it were pre 2005 and you were taking 4-AD then i would tell you yes, throw in an AI. so to answer your question, it depends on the steroid you want to run. if it doesnt aromatize then its pointless.