If I want Botox on my horizontal forehead lines do I also need between my eyebrows done to keep my forehead from drooping?

The fact that the frown muscles between the eyebrows and the mid and upper forehead muscles that make the horizontal lines are two different muscle groups is exactly why I treat these two opposing groups together. If only the forehead muscle is treated, then the opposing frown muscles which exert a pulling down motion will win out and pull the eyebrows down and with them the upper eyelids usually.  one loses the arch of the eyebrow and the upper eyelids become heavier. There are some exceptions in very young people and in some patients who only have one horizontal line which is relatively high in the forehead.The information provided in Dr. Shelton's answer is for educational purposes only and is not intended to constitute medical advice.  The information provided should not be relied upon as a substitute for consultations with a qualified health professional who may be familiar with your individual medical needs. 

Botox is a type of neurotoxin that temporarily paralyzes muscles that receive the medicine. On the face, the typical areas to inject are the forehead, glabella (the “11’s”) and crow’s feet. There are more expert injections such as in the lip, lower face, masseter muscle, neck, etc. to achieve different objectives. It is not only about the number of units injected, but where they are injected such as depth and which aspect of the muscle that ultimately dictates the final outcome.Botox and the other neurotoxins will start to take effect in 2-3 days, with peak effect at 2 weeks and will start to fade in 2-3 months. Risks of injection may include bruising, swelling, weakening of adjacent muscles.The answer to your question is no. The person injecting the forehead needs to know how to carefully do so to avoid dropping the brows.Consult with an experienced neurotoxin injector for a complete range of options that are appropriate for your situation.
Gary Linkov, MD
Facial Plastic Surgeon in Manhattan