What is the danger of using nicotine in combination with estrogen?

It can increase the risk of cardiovascular side effects like heart attack and increases risk of blood clots and strokes especially if you are over 35 y/o. In fact, most Drs wont give oral birth control pills to smokers over 35 because giving female hormones to smokers so greatly increases the risks it is not worth the risk to give the Pill to them any longer. It kind of depends on what kind of estrogen you are speaking of. Do you mean birth control pills, topical estrogens or hormone replacement therapy, but really, all estrogens carry this risk when you smoke. I know the interaction checker doesnt mention this risk but if you look up your kind of hormone, it will tell you not to smoke and use these drugs.

Interactions between your selected drugs

No results found - however, this does not necessarily mean no interactions exist. ALWAYS consult with your doctor or pharmacist.

Other drugs that your selected drugs interact with

nicotine interacts with more than 20 other drugs.
Estrogen Patches (estradiol) interacts with more than 100 other drugs.

https://www.drugs.com/interactions-check.php?drug_list=1716-0,1019-11243

Take care,