If you drop estrogen levels too low it can cause erectile problems as well as joint and immune system problems. But the effects are highly unlikely to have any permanent nature-it works by blocking the aromatase enzyme that converts testosterone to estrogen, but will only work as long as you're using it.
In fact you are likely to have estrogen rebound after letro which is why it's always advisable to run nolva post letro use.
I've heard of people having more permanent problems with aromasin, which is an aromatase inactivator, but not with letro or adex
