Nicotine is horrible, and nobody should use it.... that said, there is no actual prove that it interferes with estrogen levels to any large degree.( yes there is one or two small samplesize study that point to maybe a little) The bigger issue as to why this is often repeated is that there are still alot of unknowns about HRT but doctors do know that nicotine can definitly lead to problems. so as a way to more accurately study the long term effects of hrt they try to eliminate the most known causes of problems. Like say in a 20 year study they find that 10% of trans women were to have some kind of cancer, nobody wants to be confused as to what caused it, was it the type of hrt? Was it cigarettes? Was it diet? Ext...