Good question. However, the only way I could think of that that would be a problem is if you were blocking estrogen and worried you overdid it. In that case, back off the anti-e by half and see if you grow more without intolerable sides appearing. If so, then you know you were overdoing it. Estrogen clearly causes growth, increasing IGF yada yada yada, well-proven. If you were just taking primo, no anti-e, then you would probably have enough natural estrogen for growth. But if not, I wouldn't add estrogen to a primo or tren stack, but just take more test. Hope that helps a little. Good question though.