You can, just be VERY VERY careful with it. For example, I am about to switch from letro to another AI becaue my letro is running out. While on 500mg of test a week, I am taking a scant 0.1ml of letro. Yeah, it really is that strong. You can easily crush your estrogen levels with it, so be very careful. Also, everyone will respond differently to letro, some are more sensitive than others (as it is with every AI), but since letro is so strong, my 0.1ml might be half what you need or it could be double. You just never know.
If you do use letro, get blood work done often until you are sure how your body reacts.