Letrozole: When will I start feeling/seeing side effects?

How Fast Can Letrozole Side effects appear?
On the third day of taking letrozole, I felt as if an ice-pick had pierced my ear. The pain was excruciating and I stopped. Many painful side effects are still there - pain in ears, jaws, neck, mouth. My oncologist says this can't be caused by letrozole because it takes more than 3 days and the side effects would disappear when I stopped taking it. I've gotten some relief from increasing the meds I take for peripheral neuropathy. Does anyone have any information on how long it takes for letrozole side effects to appear? Any scientific studies too?
Thanks for any information.

@deluga, I moved your message to this existing discussion:
- Letrozole: When will I start feeling/seeing side effects? https://connect.mayoclinic.org/discussion/letrozole/

I did this so you can read the experience of others and connect with members taking Letrozole.

Like all medicines, letrozole can cause side effects, although not everyone gets them. And how long it takes for side effects to appear is different for every person.

Common side effects of Letrozole mimic menopause symptoms and include hot flushes, difficulty sleeping, tiredness and low mood. Side effects usually improve after a few months as your body gets used to the medicine.

The side effects that you're experiencing sound rare and potentially not related to Letrozole or something that is very unique to you. Perhaps a drug interaction with another drug you're taking. There is also something called ototoxicity - hearing or balance problems due to a medicine. This can happen when someone is on a high dose of a drug that treats cancer, infections, or other illnesses. Might be worth talking to your pharmacist.

What medication are you taking for peripheral neuropathy? Did you increase the dosage under the guidance of your doctor or pharmacist?

I'm taking alpha lipoic acid (ALA), a non-prescription drug in the US but a prescription drug in Europe. I doubled the dosage after the side effects from Letrozole, some of which were to make my peripheral neuropathy in my feet and legs much worse. My neurologist reviewed it.

I have been on this med a couple months. My oncologist said worst side effects his patients reported were hot flashes and joint pain. I have experienced some joint pain on occasion but Aleve did the trick. I didn’t experience the side effects until about 2 months on the drug.

Within a week I noticed wrist, elbow & ankle pain. I was surprised the letrozole was shutting down the estrogen production that quickly. I only tolerated it for 8 weeks as I progressively got so bad I was home bound from the pain & every side effect of no estrogen. I decided after going on a dr suggested break that quality of life was more important. I am 65 and have approximately 3-8% chance of a cancer return. After. 4 months I am almost back to “normal.”

I took Letrozole for 18 weeks. I had body and muscle aches, developed carpel tunnel in both hands, trigger finger and thumbs in both hands. I can't remember how quickly the side effects came on. My oncologist was not convinced it was the medication. I took a break suggested by my oncologist. I have been off any AI's since May 2022 and the last symptom, the trigger thumbs are finally going away. I have decided not to go back on any AI's for now. My oncologist would like me to take exemestane, but I am going to put my energy toward diet, supplements and exercise. I am 70 this December and quality of life is important to me. I had a double mastectomy, no reconstruction and no lymph involvement.

Thanks so much. I am now going to a doctor at an integrative health institute who specializes in cancer. She says many of her patients have pretty immediate reactions from the AIs and their doctors all say no it can't be the medication which definitely reassured me. I too am not going to take any more AIs and am trying to become a vegan, walk every day, do weights, and will probably try acupuncture for the pain. I do not like "zen" type medication but practice musical instrument now right before bed (at suggestion of Dr) which puts me in a similar alpha-wave state and helps with the pain at night. I am 81, had lumpectomy on each side (1 was triple negative on 1) and do have lymph involvement.

@deluga an immediate reaction is not likely to be a side effect from estrogen suppression but instead a reaction to inactive ingredients.

I tried anastrozole generic, then letrozole generic, then a different manufacturer, and finally brand name Femara. In my last year I did find a generic letrozole that I could take.

I really hope you share this with the integrative health folks who should realize that an immediate reaction is unlikely to be related to the active ingredient in AI's.

I got TN, a type of neuropathy (nerve in the ear) on the 3rd day and it was so painful, I stopped. My oncologist said you don't get side effects that quickly so it wasn't Letrozole. I don't believe her and will not retake it or any AI since all can bring on neuropathy in older woman. I'm 81.

@deluga I would think side effects from hormone suppression would take awhile. That was my experience.

However, there is another category of reaction that happen more quickly- reaction to the ingredients of the pill, especially fillers. This happened to me: neurological effects within minutes. It is a "sensitivity," "intolerance," or even "allergy" but not a side effect.

This actually happens to me with quite a few meds. If I switch manufacturers, often a medication will work fine without. (For me polyethylene glycol is one culprit).