I looked into this after a friend had the same question. The article I found explains that smoking, primarily the nicotine, can disrupt your endocrine system in a few key ways. It can increase the breakdown of certain hormones, which might make HRT less effective. It also affects blood flow, which impacts how hormones are delivered and used in your body. The article does a good job of breaking down the science in plain language. If you're a smoker considering HRT, it's an important read to understand the risks and what to discuss with your doctor: https://ways2well.com/blog/how-does-smoking-affect-hormone-levels-and-hormone-replacement-therapy