Why Do I Have an Indentation in Forehead After Botox?

I am not convinced that we know of the reason for this diagonal line to form. your photograph appears to show that the middle forehead is slightly puffier than the outer side of the forehead. Your line is a very sharp line as if a shelf is formed by the puffy or hanging muscle slab in the middle forehead lying against the outer forehead. Possibly, the middle forehead is relaxed from the botox causing it to sag a little because it can't elevate from the botox effect and it lies against the remaining active outer forehead that is less sagging or bulky because it is active. Further treatment in the ouer forehead may help this. It could be that coincidentally, you have developed a sleep crease from lying on your left side when you sleep pushing the outer forehead skin up against the immobile Botox-affected middle forehead. Either way, it should not be permanent.

Botox changes the dynamics of brow function.  There are essentially 2 groups of muscles in your brow.  The elevators (frontalis) and depressors (corrugators, and procerus).  When the depressors are treated to remedy glabellar furrow (the "eleven"), the frontalis remains active.  The FDA has approved botox for the glabellar furrow in 2002.  It has since gained broad acceptance among physicians for off-label uses such as crow's feet, forehead wrinkles, platysmal banding, migraines, etc.  The furrow that you have now may be due to the residual activity of surrounding muscles that have remained active while the depressors have been weakened or paralyzed by botox.  See your doctor.  Don't be afraid to address your concern.  Additional botox in the non-treated muscle may remedy this issue.  Good luck!  You could also just wait it out, but it may take 4 months to resolve.