Can filler be used in the forehead to make it look rounder/less flat?

yes fillers can be used on the forehead but should be done conservatively because this area has many blood vessels and nerves.  Best, Dr. Emer.

Yes, fillers can be used to create a more rounded (less flat) forehead, although generally this is not considered a common treatment. Hyaluronic-acid based fillers are most commonly used since they look and feel natural, and there is an antidote enzyme (hyaluronidase) to dissolve it should there be any issues. Make sure to have only a highly trained and experienced physician injector such as a dermatologist treating you to optimize natural results and safety.

Fillers can be used to make the forehead more prominent and to fill in temporal hollowness. There are many choices of fillers for these regions. Some of the most common fillers used are Restylane Lyft, Juvederm Ultra and Juvederm Voluma. These fillers tend to create more lift and volume than others, which makes them the most appropriate products to use in this area.
Keep in mind that these are off-label uses of the product and you should have these areas injected by an experienced injector. Most of these fillers will last 9-12 months.

It's hard to say whether you are good candidate for this treatment without seeing a photo or evaluating you in person but in general filler can be used to address volume loss in the forehead and temples. 

I like to use hyaluronic acid based fillers to fill in temples and forehead.  Many people have temporal hollowing as they age, adding volume in the temples will give a more smooth contour and more lifted look.  I also like fillers in the forehead to give it more smooth full youthful appearance.

Dear hxr102, 
Filler could be great treatment for flat forehead, we especially started to see this complaint for patients who used botox in the forehead for long time as the lack of muscle use reduces the muscle mass, we are using dermal filler with cannula for safe approach and it gets great results, its very important though for the practitioner to inject in the correct plan with the correct type of filler in order to get the desired effect.

Yes one can use filler but the amount required and the expected bruising with some irregularity is a factor for a wide area. Best to have a custom implant based on a CT placed which is more accurate and predictable.