What can be done for this vertical dent in my forehead, it can only be seen in certain lights. (Photo)

Hello and thank you for your question.  The
best advice you can receive is from an in-person consultation.  Based on
your photographs, you may benefit from botox.  This decision should
be based on a detailed discussion with equal input from both you
and your surgeon.   I recommend
that you seek consultation with a qualified board-certified plastic surgeon who
can evaluate you in person.
Best wishes and good luck.
Richard G. Reish, M.D.  FACS
Harvard-trained plastic surgeon

I recommend getting a consultation with a cosmetic dermatologist. Botox should be able to improve this. It takes 1-2 weeks for final results with botox. Hope this helps.

Thank you for your question about botox.  Botox does not start working for at least 10-14 days post treatment.  However, the benefits should last 3-4 months.  The amount of units needed differs from person to person.  Typically 20-26 units are needed in the glabella area ("11's) in females.  Keep in mind some may not need that much, and men typically require more units.  
It is also important to keep in mind that several treatments may be required before more severe lines soften.  Regardless, it is important to decrease the muscle movement in order to improve moderate to severe wrinkles and fine lines.

To be sure, see two or more experienced, licensed and board-certified providers in your area for a complete evaluation to make sure you are a good candidate and that it is safe for you to have treatment. I hope this helps.