How good is Letrozole when compared with Aromasin (Exemestane) and Arimidex (Anastrozole), its primary rivals? Letrozole is 10-20x more potent than Anastrozole, and about as potent (but with a slightly different mechanism) as Exemestane. It also long-lasting. Letrozole has a whopping 2-4 day half-life, and youll need to take Letrozole for approximately 60 days (with wild variance of 2-6 weeks) to achieve a steady blood plasma level (8).