Periods are a pain, literally, but we kind of like them being regular so we can anticipate when to schedule in the necessary couch-days and Netflix marathons. However, when you’re taking phentermine, your period can become irregular and your monthly symptoms may worsen due to the way phentermine affects your hormones. Here we explain how phentermine affects your period, and how to cope with these changes.

How Phentermine Affects Your Period

Although it is not listed as a side effect of phentermine, anecdotal evidence shows that many women experience changes to their period when taking the medication. Discussions on the phentermine forum indicate that the following changes have all been experienced by our forum users:

  • Missed periods (no menstrual flow for over 40 days)
  • Late periods (5 or more days overdue, according to usual cycle)
  • Early periods (5 or more days early, according to usual cycle)
  • Heavier or prolonged periods
  • Lighter or shorter periods
  • Spotting or intermittent bleeding between periods
  • Infrequent menstruation (only 4 to 9 periods in a year, medically known as Oligomenorrhea)
  • Frequent menstruation (more than 17 periods in a year, medically known as Polymenorrhea)
  • More severe symptoms – cramps, premenstrual syndrome (PMS), nausea, dizziness,

Essentially, these changes to your period all indicate irregular menstruation, whereby your monthly cycle and the symptoms which occur alongside it are different to the menstruation cycle that you used to experience before taking phentermine.  While these changes can be very concerning, we hope that knowing that many other women have experienced the same symptoms when taking phentermine helps to put your mind at ease – which is why the phentermine forum is such an important source of information and support for many phentermine users.

Why Does Phentermine Affect Your Period?

In much the same way that phentermine has been indirectly linked to hair loss, it is thought that the changes to your period occur alongside phentermine due to the way that weight loss can affect your hormones, which in turn affects your monthly cycle. Dramatic weight loss, which phentermine users often experience, causes a drop in the amount of estrogen in the body, as the release of estrogen is partially based on the amount of fat cells in the body. This decline in estrogen production can then cause changes to your menstrual cycle, resulting in irregular periods and can even cause your periods to stop completely.

An alternative theory is that the way in which phentermine stimulates your sympathetic nervous system to trigger the ‘fight or flight’ response can result in a reduction in the release of reproductive hormones, which would explain why some women taking phentermine may experience fewer periods than usual. However, irregular periods, heavier menstrual bleeding and spotting can also be explained by the way that phentermine in known to increase your heart rate. A faster heart-rate means increased blood flow and more build up in the ‘spiral arteries’ which support the endometrium of your uterus, which then degenerates to cause the onset of your period.

Another reason for an abnormal menstrual cycle is poor nutrition as a result of not eating a varied enough diet, possibly due to a lack of appetite while taking phentermine. Regardless of your weight or the amount of exercise you do, a lack of iron, zinc, and healthy fats in your diet can all cause changes to your period and an increase in the severity of menstrual symptoms such as cramps, dizziness and nausea.


How To Reduce These Symptoms

If you think that poor nutrition is contributing to your menstrual irregularity then taking a comprehensive multivitamin such as Phen Vites alongside your phentermine prescription will ensure that you’re getting all the nutrients you need for a healthy body, and this will also help to ease the symptoms of irregular periods, menstrual cramps, and other symptoms. In fact, making sure to get all the vitamins and minerals you need to keep healthy will also boost weight loss, as your body needs a variety of nutrients to burn calories and keep your metabolism going. As well as taking a multivitamin, you should try to eat a varied diet, being sure to include healthy foods rich in iron such as leafy greens, lean red meat and pumpkin seeds to help reduce some of these menstrual symptoms.

If you are sexually active and able to become pregnant then you should be using some kind of birth control as phentermine cannot be taken by women who are pregnant or trying to become pregnant. If your choice of contraceptive is an oral birth control pill then taking the pill continuously is one way to limit severe periods and other symptoms that come with menstruation, such as PMS and cramps. This method is often recommended by medical professionals to women with heavy or painful periods, but they do advise that you should have at least four periods a year to maintain the correct balance for future fertility. So, instead of taking the placebo pills or skipping your pill for a week, just move onto your next packet, but just be sure to give yourself a break from your birth control pill every couple of months. Avoiding having a period every month means that you will be able to get some relief from the effects that losing weight with phentermine can have on your menstrual cycle. Not to mention, skipping several periods can also help you to avoid the other symptoms of that time of the month, such as weight gain, bloating and cravings, so staying on track while taking phentermine is much easier too.

Lastly, if your period is often irregular and you have had difficulty losing weight in the past, as well as other symptoms such as fatigue, cramps, constipation, fertility problems or acne, then you may be suffering from an undiagnosed health issue, such as thyroid problems or Polycystic Ovary Syndrome (PCOS), which both affect the body’s hormone levels. Although losing weight with phentermine can help to relieve some of the symptoms of these conditions, it is advisable to be tested for these conditions, if only to rule them out as possibilities.


Although coping with irregular periods and more severe symptoms can be difficult, getting healthier by eating a more varied diet and losing weight with phentermine will help to resolve these issues in the long term. The good news is that when you get closer to your target weight your body should adjust to the new amount of hormones, and your periods should become more regular and the symptoms less severe.

Have your periods changed since you starting taking phentermine? Let us know by commenting below.


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24 Responses

  1. The first month I was taking it (May) I noticed my period was longer than normal. This month I was supposed to get it 6/14 but have yet to get it. I took a pregnancy test on the 17th that was negative and I have all the symptoms of my period coming as well has using protection in the form of condoms without any complications.

  2. Yes, my period has been severely impacted by phentermine. I initially lost 25 lbs in about 5 weeks with it. I have been bleeding heavily ever since – for over four weeks now with intermittent heavy large clotting for days at a time. I’ve had my cycle for 29 years (got it early at 10) and I’ve never had anything like this. I have had heavy periods before and I have several fibroids but they are not large yet never experienced anything like this – especially with the large clots for days. I’ve had to increase my iron intake because I’m getting weak and exhausted. I keeping reading it’s “a normal” side effect but when will it end?

    • Hi Amanda,
      What you describes sounds more serious than most people experience on phentermine – have you spoken to your doctor about this? The clotting sounds concerning, as does your significant loss of blood.

  3. Yep it sure did. I am 52 and haven’t had a period in over a year but after I started taking phentermine for a couple of days my period is so heavy its scary. I’m going to stop taking the Phentermine today but on a positive note it stopped my hot flashes and night sweats!!!!

  4. I’ve been on Phentermine for nearly 3 months now and I have not gotten my period. I took a pregnancy test and was negative. I do have all the symptoms of a period but nothing. But the medication is working so far.

    • Hi Sabrina,
      I wouldn’t be too worried, it seems that these kinds of changes are quite typical among women taking phentermine. Just be sure to eat a nutritious balanced diet and your period should return when your body gets used to the drug.


    • Hi Kayla,
      Late periods are quite common for many women taking phentermine so this is not a reason to be worried. If you are sexually active and could become pregnant then it is advisable to use contraceptives of some kind while taking phentermine. If you think you could be pregnant then you should do a pregnancy test and stop taking phentermine immediately if you are pregnant. With the bloating, this may be related to your late period and water retention, so be sure to drink lots of water and to avoid foods which can cause bloating such as salt, fizzy drinks, cruciferous veggies, dairy and chewing gum.

    • My cycle is 6 days late and I’m having all the symptoms of it coming such as bloating, headaches, and cramps. I haven’t taken a pregnancy test because the last time I had sex I was not ovulating. I started taking phentermine on July 7. So what I’m reading about your period being delayed can really be happening. Please help somebody!!

  6. I started phen on June 21 of this year as well as my period started that day. Period started heavy and ended in 6 days. After my trial phen of ten days I went a week without it. Between 6/30-7/6 I was sexually active and unprotected. I have an allergy to whatever is on the condoms and a latex allergy. I also cannot take birth control because it causes irregular heavy bedridden bleeding. I have been tested for everything ultrasounds the whole nine and I’m just fine down there. I went from 7/5 to 7/11 without taking the phen. I experienced some spotting on 7/6. On 7/12 I started back on the phen. I experienced spotting again on 7/19 a clear discharge on 7/20 and then in the afternoon heavy bleeding occurred. I had to leave work early and I have not been back to work. I have been experiencing cramping in my abdomen lower back glutes and thighs and legs. I’ve experienced nausea and vomitting as well. Even fatigue which I do not see how is possible because usually I’m awake while using phen. I am praying that it will lighten up by 7/25 so that I can go back to work. It is very heavy very red and lots of clotting. I experienced this in the past with birth control. A typical period without birth control and of course phen is 6-8 days medium to light flow with no abdominal cramping no nausea no dizziness no vomiting.

  7. I started Phenteime on June 26, my cycle was heavy. I’ve now been on it for approximately a month and a half, and now have had spotting. No nausea or other symptoms, and I’m super happy with my weight loss!!

    • Hi, I have the same problem, did you find any remedy to your problem with your prolonged period? I still have mine going and need it to stop. Have you tried any supplements?

  8. I was taking birth control pills before starting phentermine. This was to regulate my periods because they were too heavy. But now that I started phentermine they are not working my periods are heavy again. What can I do? I gained weight with the pills and got dark spots on my face. That’s why I decided to take phentermine. I have lost 38 pounds in five months. But now the problem is with my periods, I have anemia.

    • Hi Veronica,
      We think that it would be best to talk to your doctor or OBGYN about the problems with your period and how to manage them whilst taking phentermine.

  9. Started taking Phen one week from today and my period was supposed to come that Friday and it never did. Second week today, I have cramps like my period is coming but it has not. I feel a little nauseous and dizzy. I also feel bloated and I don’t think I have lost weight. I did take it once before like months ago and I did lose weight.

  10. My husband and I have been trying to conceive for over a year. I have had tests taken to check my thyroid and for pcos and they all came back healthy. However, my periods are so irregular, I never know when or if I am ovulating. My Dr. Suggested I lose weight, so now I am trying phentermine. I have been on it for 5 days now and have lost about a lb. a day and have been eating and drinking healthily. This morning, my period (which has been spotty for months) decided it would show up. I bled everywhere, it woke me from my sleep. It seems as if my blood has thinned? There were large clots. I am hoping that my body is trying to flush out the lining that has been building up due to lack of flow with my period. Here is hoping for regular future periods and healthy weight loss so that we may soon try for a baby. If this bleeding persists, should I discontinue phen?

    • Hi Kristina,
      As the article explains, irregular periods are common on phentermine, both in the sense that the user will often experience changes to what they used to expect from their period, but also that the period can come more or less often than might be considered in the normal range, and be particularly light or heavy. I would say that the explanation you’ve given as to why you think this period is heavy and with clotting sounds quite likely as you have previously experienced infrequent periods and spotting, so there may have been some build-up from previous months. I would discuss this with your doctor as he or she will be able to offer you advice about what to do if your period becomes more of a concern, as it may not be necessary to stop taking phentermine. Lastly, to ensure that your blood loss doesn’t affect your health and weight loss due to low iron levels, it could be a good idea to take iron supplements or make a point of eating iron-rich foods, as we explain here:
      I hope that helps!

  11. I have taken phentermine for 2 months and have had a large amount of hair loss, so I have quit taking it. I just read about the hair loss. I am now taking biotin tosee if I can get my hair to grow back

  12. I have been on the phentermine for 2 months. I have always had irregular periods so that is nothing new for me but I have had ovarian cramps for 3 weeks and excessive ovulation and discharge for the 3 weeks too. Side effect??

    • Hi Melanie,
      This is something we haven’t heard of before as a side effect so it could be a good idea to see your doctor about this, even if just to be reassured that it’s nothing to worry about.

  13. My period hasn\’t stopped after a week of taking phentermine. I’ve had my period for about 6 weeks now. Should I take iron and zinc supplements to help? I decided to stop taking phentermine 2 weeks after I noticed I still had my period. How do I regulate my hormones? What vitamins should I take? If someone knows please help. I don’t have access to any doctor or hospital at the moment, but I have access to a store where they sell vitamins. Thanks in advance!!!!

    • Did you figure anything out?

  14. I have the nexplanon implant in my arm for birth control and it has caused me to not have a period for nearly a year. Then on my second month on phen it starts and I’m super emotional and ill (one of the main reasons i wanted the implant) I just wish I could handle my emotions.

  15. I have PCOS. I took duromine to help me start to lose some weight because I really would like to get pregnant again.
    1) I have regular periods… like clock work… but I get terrible cramps and hectic bleeds and can barely walk for 2 days of my 7 day period. I throw up and get headaches… I do see a doc yearly…
    2)I have been like this since I was a child when my periods started.

    I stopped taking duromine daily… slowly weaned myself off it and found I have kept the 14kg I lost off with out any hard work.
    But … when i have my periods and I am in agony… I find taking a duromine takes the cramps away… it is almost a miraculous, within the hour effect… I go from sitting and rocking myself in pain to being normal.
    It doesn’t seem to thin my blood… it seems to ease the headaches… I am pretty much using duromine lately only for those 2 days of hell.
    Painkillers don’t have this effect on me…it just dulls the pain, and anything with aspirin in it will make my nose bleed at this time. I will discuss this with my doctor at the end of Oct.

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