I'm using letrozole for 5 years because of breast cancer. Can I use face fillers?

Letrozole is an aromatase inhibitor used for treating breast cancers which are hormonally responsive.  Fortunately, there is no interaction between standard HA fillers and letrozole, and none of the fillers will impair its effectiveness.  So you can go ahead.

Thank you for your
question. Hyaluronic acid
does not interact with Letrozole, and will not increase your risk or relapse of
Breast Cancer. You can have facial fillers while taking Letrozole, because
fillers are comprised of hyaluronic acid which is a natural substance that is
already present in your skin. Hyaluronic acid is extracted and reformulated
which has now become one of the most popular kinds of injectable fillers.
Additionally, fillers are a great way to rejuvenate the skin by volumizing and
smoothing out fine lines and wrinkles. At my practice I understand that everyone’s
“ideal” look correlates to their facial structure, which is why a proper
in-office procedure is always required. Here is a tutorial of how I approach
and perform fillers at my practice. Best, Dr. Ostad.

There should not be any issue with facial fillers and taking letrozole.  If you have any specific concerns, you can always discuss them with your physician/surgeon.  Best wishes.