Tamoxifen and Running: A Little-Known Side Effect

Have you experienced any side-effects while taking tamoxifen? Did it take you a while to figure out that it was tamoxifen causing your symptoms? Have you experienced this with any other medications? Did you feel inferior, insecure and incapable when experiencing these side effects? I did. And this is my story of a little-known side effect of tamoxifen.

What is Tamoxifen?

Tamoxifen is a drug used in the treatment of estrogen responsive breast cancer. It blocks the action of estrogen, a hormone, to stop growth of breast cancer cells.

Tamoxifen is the cornerstone for hormonal treatment in younger breast cancer patients that are not yet in menopause. It has a good success rate for preventing cancer recurrence.

Patients are usually on this medication for 5 to 10 years. However, it is notorious for having side effects such as blood clots, liver problems, intestinal issues, weight gain and others.

One side effect, that I experienced, is not well understood or reported, but many people on tamoxifen experience it.

I started running before tamoxifen

I started hormonal therapy to prevent breast cancer recurrence in 2014. After chemotherapy and radiation, my oncologist put me on a medication (goserelin) to suppress my ovaries and a drug (exemestane) to prevent my cancer from responding to any estrogen.

Except for bone pains, I experienced very little side effects to these medications. Running seemed to help lessen the bone pains, so I kept running. I started by running 5 km, then gradually progressed to running 10 km, 10 miles and half marathons.

My first half marathon was during the Sault International Festival of Races in the fall of 2015. I ran it in a 2:37:20 time. Not a great time, but not bad for my first one. Surprisingly, I won first place in my age category. I wanted to run faster and was all set to work harder.

I signed up for a training program and wanted to run my next half marathon in under 2 hours and 30 minutes

Life was good during the first few months of tamoxifen.

Then my oncologist recommended that I go on tamoxifen. He was concerned that my estrogen blood profiles were not convincing enough that I was actually in menopause. In order for exemestane to be effective against breast cancer, I had to be in menopause. So, wanting to do everything I could to fight my cancer, I switched to tamoxifen.

I started training again in January of 2017. I had already been on tamoxifen for numerous months. My oncologist said my hormonal profile was good. I felt healthy and I was still running. I did not have any more bone pains. Life seemed good.

Then I started to have problems on tamoxifen.

A few weeks into my next half marathon training program, I was not improving. My fellow runners were all improving; getting faster and going further. I was not able to keep up with my running buddies. It was very frustrating to be putting all the work into my training, but go nowhere.

It got worse.

A little over half way through my program, I was getting slower.

It was becoming harder to run the distances that I had already run before.

I wasn’t recovering well from my runs.

My muscle soreness that lasted for days.

I seemed to be going backwards in my training.

My 5 km finishing times showed a steady decline.

My problems on tamoxifen got worse as I continued to run.

Then I ran my next half marathon.

I ran the famous and sold out Garry Bjorklund Half Marathon, which is part of the Grandmas Marathon Weekend in Duluth, MN.

Spoiler alert: It did not go well.

At the half way point, I started to get some leg muscle cramps. They got worse as I ran.

Two thirds of the way into the race, I couldn’t run 5 minutes without having a sudden muscle spasm that would bring tears to my eyes. I had to walk much of the last few miles.

I was determined to finish and I was determined to run across that finish line. It wasn’t pretty, but I did it. I finished the race with a chip time of 3:07:22, far from my dream of a sub 2:30:00 half marathon time.

I collapsed in pain after I crossed. All my leg muscles seized up.

Why was this happening?

I didn’t understand why this was happening to me. So, as I always do, I searched the science journals for answers. I came across a research article that validated what I was experiencing.

Hormonal therapy with tamoxifen and raloxifene are listed as causing muscle cramps. It seemed that tamoxifen was the likely culprit for my declining running abilities and muscle cramping.

Unfortunately, there is not a lot of research on the details of why this happens. And it is not clear why this happens in some people taking tamoxifen and not others.

I have more questions about it than science has been able to answer so far.

Tamoxifen has saved lives.

I am not saying tamoxifen is bad. Tamoxifen has saved so many lives. It has reduced breast cancer rates by a third.

You shouldn’t let tamoxifen deter you from running or doing the exercise you love to do.

Exercise in conjunction with tamoxifen has been shown to reduce mammary tumor burden. You should use ALL the tools in your cancer fighting tool box, including following your oncologists advice, healthy food and some form of exercise.

Related Articles:

Manage Tamoxifen Side Effects with Cardio Exercise

Do’s & Don’ts of Eating While Taking Tamoxifen

The Best Exercise for Cancer Survivors

Why did I tell my story?

The reason I am telling you my story is that if you are struggling and frustrated that you are not improving physically, you are NOT weak, feeble or crazy.

I thought it was me. I started thinking I wasn’t strong enough and that I wasn’t cut out to be a runner. My experience almost made me quit running. I am so glad that I did not quit.

This experience has made me rethink why I run.

I don’t run to compete or to win any races. Instead, I run for health, both physical and mental health.

I run to clear my mind and reduce stress.

I run to confirm that I am strong, that I can do anything I set my mind to do.

And, I run to have fun with my running buddies, who are some of the best people I know.

And all these reasons for running have absolutely nothing to do with pace or finishing time.

Be active for health. Do it for you. Just keep moving, no matter what level you are at, no matter what obstacles you need to overcome. Just don’t quit.

My story does have a happy ending. My oncologist switched me back to exemestane. In the Fall of 2018, I ran the Toronto Waterfront Half Marathon with a time of 2:27:40. I did it!

I also successfully ran further than I ever have before. In October 2019, I ran my first marathon at age 52.

Have you experienced side effects on tamoxifen? If you feel comfortable sharing, please let us know what you experienced in the comments below.

November 2, 2020: Update to the science behind Tamoxifen and Running:

I have been keeping my eyes open for any new research that explains what I, and so many other runners, have experienced while taking tamoxifen. I haven’t found much for this specific issue.

Interestingly, scientists are using tamoxifen to induce muscle problems in genetically altered mice to study exercise physiology. The details do not apply to our situation, however.

Tamoxifen is also being studied for its positive effects on muscular dystrophy. It seems to strengthen muscle contractions in these patients.

This research tells me that tamoxifen has an effect on muscle. But I haven’t come across any new research studying tamoxifen effects on athletic abilities of breast cancer survivors.

I am as lost as ever to explain why my running was affected so negatively while taking tamoxifen. But so many runners (and I am talking a lot of women!) have reached out to me to say they have experienced the same thing, or similar issues. It is not an isolated problem.

I will keep looking for answers. If I find any, I will post another update.


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  1. Thank you for sharing your story which I came across after googling ‘best exercise when taking tamoxifen’! Like others have said I could have written the same story. I have been on Tamoxifen for coming up to 4 years. I have always run and have found it harder since being on Tamoxifen, the side effects seem to come and go….bone & muscles aches, hot flushes, tiredness….I think I manage it well and I’ve always kept on exercising and running. However in recent training for a marathon my times have slipped, it all seems so much harder and I struggle to recover from long runs. Do the effects get worse after several years or is it that I’m closer to menopause age, who knows?! I definitely agree that taking it back to running to enjoy it, slowing down, looking around and being grateful that I can still run!

    1. I am so glad you are still running and enjoying it. Yes, I found it got harder over time too, unfortunately.

    2. I have been on Tamoxifen since March 21, 2022. I had cut my running back before my mastectomy since my knees are getting bad. However, I could still run a few miles at 9:30 pace. Now, every run is in the 11-12 minute pace. I could blame it on getting old since I’m 65, but this is a major slowdown. At least I’m not imagining things. Hopefully, someone will figure out why this happens.

        1. I have an in-process update. I was diagnosed with Afib. I decided to take my resting pulse and it was between 75 and 95 when it normally was in the 50’s. I had an EKG done by my PCP, then by a cardiologist that confirmed the results. So, this definitely affects the pumping of blood and ability to get enough oxygen into the system. The cardiologist said it has nothing to do with Tamoxifen. It could have happened from the surgery or randomly since I’m getting older :-(. I’ll let you know how my cardioversion goes in a few weeks. At least I have an explanation.

          1. It is great that you have some answers now. I wish you speedy healing. It is always good to address any symptoms you are experiencing with your doctor.

  2. I came across your post and wanted to thank you so much for sharing. I am at 4 and 1/2 years on Tamoxifen and although I have been tolerating it fairly well (so tired of the hot flashes though), the aches and muscle soreness have gotten worse during this time and it’s so frustrating. I bike, walk, and lift weights…always do my 150 minutes a week as recommended…but it’s hard. It takes so much longer for soreness to go away and if I overdo it in one area I am sore/in pain for days. Plus just feeling too tired to exercise is really annoying. I am looking forward to being done with Tamox. in in 6 months…I will see it through. Almost there! Thanks again for sharing. It is comforting to know that I am not the only one although this really sucks for all of us. Take care and best wishes. 🙂

    1. Thank you for sharing your story. There are so many women that have experienced similar side effects. It helps to know that you are not alone in this struggle. I am glad you will be off tamoxifen soon. It takes some time to recover from this, so in the meantime, go easy and take care!

  3. I’m so glad to have found this website. I’ve been a keen runner for 15 years and even managed to keep running throughout chemo. I started taking tamoxifen 10 months ago and noticed ankle and calf pain first thing in the morning and especially whilst running. I’ve tried physio and acupuncture without success, I now struggle to do 5k because it’s so painful. I cycle and swim regularly with no issues. But running is my passion and I can’t imagine not doing it. I have now given myself a month with no running at all to see if this helps x

    1. I am glad you found Pink Ribbon Runner too. I hope you find lots of good information here to help you. And may rest heal your ankle and calf pain so that you can keep running.

  4. I cant believe it I thought it was just me. After mastectomy, chemotherapy/radiotherapy I started tamoxifen and then C25km. Although not a runner before cancer I had always played sport such as hockey to a reasonable level. Now 3 years on, I’m a keen runner 3/4 times a week (around 25km), however my speed is atrocious, I sweat and sweat, and my heart rate is crazy with any harder effort e.g. hills/ races….My legs always ache pretty constantly and I have hot baths most evenings to help things. I thought it was just me as I had no previous running pace to compare previously with.
    I tried anastrazole as an alternative as my hot sweats are just silly on Tamoxifen but it gave me horrendous migraines. The good news is I love running, Im a survivor…. but Id still like to be a bit faster and stronger😃

    1. You are a survivor! I am so glad you discovered running. Yes, it is slower and harder after cancer. But enjoy it for what it is. It is a powerful tool to help heal both body and mind. Speed and strength come over time. Just keep running! 🙂

  5. Thankful to read your post. I did a half Ironman last fall and was plagued with muscle spasms throughout the run. I have now accepted that I may have to stick to shorter events. I too saw a dramatic decline in performance. It has taken me two years to get to the point where I can run sub 9 min miles. I was previously able to run sub 8 on a typical training run. I have to work hard to stay hydrated. My heart rate goes unusually high the first mile of a run. I’ve learned to start slow and that seems to help. I have found that while my pace has slowed, I am able to go for long runs and bike rides provided I drink throughout. I sweat an embarrassing amount. I agree with PRR in that you must find a way to keep moving.

    1. Thank you for sharing your story. I am glad you found a way to work through this and keep moving.

  6. This blog post made me realize that I’m not alone! I run regularly and generally do a yearly half-marathon but my training pace has been so slow since I started taking Tamoxifen about 10 months ago. I can’t seem to speed up at all; I’m like lead. I did a search for “running + Tamoxifen” and found this post. It can’t be all in my mind!! 🙂

  7. Wow what a relief I am not alone! I am training for a marathon (Paris) and am really struggling. My calfs keep cramping up and are so tight! Anything more than 10 or so miles and I’m really struggling. I thought I was just getting older and less fit but I have run 4 marathons before without all this pain! Pre Tamoxifen.
    I ran 10 miles today and I had 17 on my training plan but no way was it going to happen today… I came home very disheartened and had a long soak in the bath. I then googled it and found this! I’m stretching loads, do yoga and have sports massages but still suffering so think I will have to just slow it all down and maybe even run walk the Marathon.

    1. Do what you need to do to finish….but never quit! YES! Run/walk if you need to. Whatever gets you to the finish line.

      1. I guess we all end up here looking for support and answers. I have only started running in the last six months so a lot of fitness is to do with being new to the sport but something is just not right 🙁 I constantly feel like lead.

        1. It is not easy, that is certain. But you are doing it. Every step you take lowers your risk of recurrence and helps with side-effects. Take pride in your accomplishments. Crossing the finish line is all the more special.

  8. I am so glad I found this! I ran half marathons consistently before being diagnosed at 31. After surgeries and radiation I was still able to maintain my pace. But then I started Tamoxifen and every run felt so challenging, my legs felt like lead at times, and no matter how many speed workouts I did I couldn’t get back to where I was. I had to change my mindset for why I run – I don’t run for PRs but to keep my body healthy. I ditched my tracker. It was freeing. But I still get sad when I run half marathons and I’m so slow. This post reminded me that it’s not me, I’m not a failure, and I should be proud I can still run 13 miles.

    1. I am proud of you for continuing to run. I know how hard it is to struggle like this. Going backwards despite the all the time and effort put into training. I like your positive spirit. Keep running!

  9. I’m so glad you shared your story! This is exactly what I’ve been experiencing. I used to run full marathons at a 9:00-9:30 pace. Now I struggle to run 3 miles at an 11-12:00minute pace. At first I thought it was just because I had gotten out of shape with no exercise between numerous reconstructive surgeries so I trained hard but was getting nowhere. In fact I was getting slower. It was/is so frustrating. On one hand I’m relieved to know that the reason isn’t just me becoming weak minded but on the other hand I’m sad to think there’s nothing I can do to get faster. I guess the next step is to change my mindset and learn to enjoy running just for running and not the PR’s. Now that will be a challenge!

    1. I thought I was going crazy too. Thought it was me, like I wrote. But I have had so many reach out with the same experience. This is a real thing. I am so sorry you are experiencing this side effect. Running marathons at that pace, even with the break, shouldn’t leave you that out of shape. Your fitness was there. Should have been relatively easy to come back with effort. I will keep looking out for more research on the subject… but to date, there is no more, unfortunately. If I find any, I will post again. I am glad you are not giving up. Yes, run for fun. The pressure to perform is lifted now. Just run! <3

      1. Thank you for writing this article. I Googled running while on Tamoxifen because I was determined there was a connection and I was desperate to know I’m not alone. It’s hard knowing you can’t be the runner you used to be but you are right – it’s still worth it!

  10. Oh my goodness! This is me!! I have asked my doctors and my trainer, but I thought I was going crazy! While I have gotten physically stronger since going on Tamoxifen, my running has completely stalled out! And I get horrible muscle cramps! Thank you so much for writing about this!!

    1. You are not crazy. Many women have reached out and told me that they have experienced similar side effects. I even chatted with a cancer researcher, trying to convince them to study it. I hope the scientific community will look into this phenomenon for us. I don’t know if there is a solution, besides switching medications?

  11. I’m late to your post but wanted to share my experience so far. I’ve run most of my life, got dx’d with breast cancer last April and have started tamoxifen as of three weeks ago.

    Since about day 10 of tamoxifen, I’ve had an increasingly hard time keeping my heart rate in the aerobic range, it spikes to threshhold and max at paces that are barely a walk. Since my normal training runs average about 10 min per mile, it’s very weird to see my heart rate spike when I’m running a 12 or even 14 min mile.

    I also started on Prolia before the tamoxifen but nowhere can I find any mention of heart issues as side effects. Same with radiation. I’m thinking it’s the tamoxifen and if that’s the case, I’m not going to take it.

    1. I’m so glad you found this post. I am sorry you are having issues. I have heard this from so many women now. The science just isn’t there to explain the side-effects we are experiencing with this medication. I hope that researchers will find out why it affects our muscles, including possibly the heart muscle (?) like this. Talk to your doctor. There are alternative medications for hormone responsive breast cancer. I wish you all my best. Consider joining our new Cancer Survivor Healthy Living Network on Facebook to keep in touch?

  12. Thank you! Thank you! Thank you! I was an on-again off-again runner before tamoxifen, but got serious about it more recently and have been completely frustrated by my “progress.” I kept thinking I was doing something wrong, but couldn’t figure out what. I have another 18 months of the meds and now I also have hope that my running will become those things I wish for (modest wishes: little faster and easier) once I stop taking it.

    1. Once off tamoxifen, I gradually came back to run faster and further. Any muscle cramping now is only associated with extreme exertion (the “normal” kind of muscle cramping). There is hope that your running will be all that you wish for. All my best. Keep on surviving!

      1. Thanks for this, I thought I was crazy and my husband thought so too. I was about to give up running. So there isn’t anything I can do to get back to myself as long as I am on Tamoxifen? I used to run relax at a 7-8 pace and I can barely make an 8 mile pace now, can’f feel the same endurance I had and I sweat like a pig!

        1. I keep looking for the answer to your question. My last search of the science journals didn’t help. No one has published anything on it. I asked a cancer researcher that I know if their lab could study it. They were interested, but I don’t know if they went in that direction. If I ever find the answer, I will be sure to write another article on it. There are so many people experiencing this side effect. And with all the research saying that running and cardio exercise helps breast cancer patients, it is so frustrating that tamoxifen does this. I am sorry you are struggling. It is okay to run slow…better for fat burning, anyways. Just enjoy your runs. Don’t worry about pace or time. Difficult to do, I know… been there!

  13. I just started on tamoxifen about a month ago. I noticed that my running has been slightly affected, especially running uphill. I will be monitoring how this affects me overall and discuss with my oncologist if there’s too much negative side effects. I’m glad you are doing well! Good luck on your future races! BTW, I live in the Sault, so I thought that was cool to see that it was your first 1/2 marathon.

    1. First, I am sorry to hear you have breast cancer. Second, I am glad you found my website and are running. Keep on running! Despite my frustrations with Tamoxifen, running was such a tremendous help in my survivorship. Thank you for you kind words and comments. All my best!

  14. Thanks for sharing this Tricia – I think it’s important to share this story as a reminder that everyone should be mindful of the medications they’re taking, regardless of what their illness is. Good for you for taking the initiative to do your research, and I’m glad your doctor listened to you.

    1. Thank you for the comment. Medication side effects can be debilitating. Yes, you need to discuss any side effects you are experiencing with your doctor. In most cases there may be other options for you.

      1. Thank you for telling your story. I have been on tamoxifen for 8 months and I am having a hard time keeping my running going. Before tamoxifen I trained at 8 1/2 minute miles. Now I struggle to do 11 minute miles. And it is hard work. I am also getting slower as time goes by.

        There was much that I recognised in your story, particularly muscles taking longer to recover. One thing that I experience which I haven’t heard anyone else mention is that I often feel nauseous when I am running – is this something that you have ever experienced?

        1. Hi SH. Thank you for reaching out. I am sorry you are struggling with running. It is the tamoxifen. And it is very frustrating. But there are so many benefits to running, so please don’t quit. Keep trudging forward. Run for enjoyment now. When I first started struggling, I got upset. But then I realized that running for the pure enjoyment was something I enjoyed even more than the triumphs of racing and doing well. It takes a different mindset…but try not to let cancer rob you of your running. It takes so much from us as it is. I, personally, was never nauseous when running on tamoxifen, but it can do that too. I have heard this from others.

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