Depends on what you are using it for. Arimidex will prevent sides from occuring. Letro reduces problems that have already occured as a result of estro sides. While Letro is very strong and will work as an AI if I were you I would run Aromasin or Adex if problems like gyno have not already appeared. Save Letro for last resort. BTW I am on Letro now so I am pretty familiar with it.