I am really glad that I found this thread because I now know that I'm not the only one going through this. I am NOT a doctor, but I will be one day.. so far, I have a degree in both health sciences and psychology and its funny- I learned about drugs and the effects they have on the body, both male and female, almost every semester and I never thought it would happen to me. I have been using for slightly over a year now and my menstrual cycle just completely stopped about 3 months ago. At first, I was FREAKING OUT. I thought for sure I was pregnant because my boyfriend and I constantly have unprotected sex. I had to have taken 30 pregnancy tests throughout the first 2 months. I would buy a pack of two almost every day just to put my mind at ease by seeing the negative symbol on each test. I even went to the doctor a few weeks ago for a blood test (not sure why it took me 3 months to do that but whatever).
Then.. I thought back to what I've learned in school. Opioids, especially dope, don't just fuck with the dopamine and seratonin in your brain.. They fuck with every chemical process in your body. (And by "fuck with" i pretty much mean "slow down") Which obviously includes the hormonal chemistry in both men and women. Women just have more physical symptoms because they are the ones that have the most physically visible hormonal processes of both sexes. Dope suppresses the estrogen and progesterone being made in your body. So, if you aren't getting enough estrogen and progesterone, your brain isn't going to be able to tell your body to menstruate every month. Since it isn't completely stopping the hormones from being manufactured in the brain, just slowing them down, you might get your period every so often, but go without having it for months. In my case, I haven't gotten it for 3 months straight. I keep hoping that this month will be the month that it happens.
Although this isn't the healthiest thing to have happen to you, the good news is, when you stop doing the drugs, you'll start getting your period again! And you are most likely not pregnant, that's good news as well lol. Also, you will probably still be able to get pregnant in the future (after quitting dope, of course) but it may be slightly harder for you than for someone who has been drug free their whole lives because, even after quitting and building up your estrogen/progesterone levels again, your levels will still not be as high as they were before you started doing dope. So if you've decided to quit, and you're trying for a baby, just keep trying.. And maybe try a little harder [which isn't much of a terrible thing to do in my opinion

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So, don't worry! Even though we've all gotten ourselves into a mess that scared the shit out of us at first, your period is not gone forever, and there is still a chance we all can get pregnant once we are back to our normal, healthy, drug free selves again!
I hope this helps with your questions as to what is going on in your body because of your drug use. Like I said, I am not doctor (yet) but this stuff is factual and after freaking out as much as I have over the past few months I would love to know that I was able put the minds of other people in the same position as me at ease!