How Many Botox Units for Horizontal Forehead Lines?

Thank you for your question.  Botox for the forehead requires careful assessment of the brows and eyes.  Most people with strong horizontal lines across the forehead have them because of anatomically low brow, deep set eyes or heavy lids.  Injecting Botox into the forehead may do a good job reducing them, but if done so with too large of a dose will cause a very uncomfortable downward heaviness on the eyes.  A good practitioner will approach your treatment with this in mind, and will also inject the muscle group at the brow (glabella) to provide needed counterbalance while relaxing those horizontal lines.  In my practice, patients with this presentation will be assessed, and injected on the more conservative side in the forehead (about 8-14 units) while also treating the glabella, and asked to follow up in 2 weeks. At that point, a few more units may be added if warranted.   

There is no absolute number of units of Botox a patient should receive to treat a specific number of forehead wrinkles.  In our practice, the range of units that we use can vary from four to thirty-four, but this does not really tell the story. Not merely the dosage of Botox, but the placement and number of injections in which muscles, and which portions of the muscles, needs to be individualized to the anatomy and aesthetic goals of each patient.The treatment of forehead wrinkles with Botox injections is really a double-edged sword.  Many patients have the transverse wrinkling of the forehead that they see, because they have brow ptosis, i.e., the soft tissue of the brow has descended to some degree, and the patients need to fire their frontalis muscles on a virtually continuous basis to elevate the overlying soft tissue, so that their field of vision, especially on upward gaze, is not diminished.  This activity causes wrinkles to form in a direction perpendicular to the vector of pull of the paired frontalis muscles.However, when a physician injects Botox into the frontalis muscles underlying these wrinkles, he is causing a weakening of those muscles, and thus a diminution of their ability to elevate the ptotic soft tissue of the brow.  Inject too much Botox in the wrong places, and you will have actually exacerbated, or, in some cases, caused, brow ptosis in your patient.In our practice, we make a point of first supporting the brow by injecting Botox in the periorbital muscles whose vectors of pull naturally counteract that of the frontalis muscle.  Once we have done this, we are extremely deliberate, precise, and conservative, in both the dosing, and the placement, of Botox injections in the forehead.  Remember too, that in addition to causing ptosis of the brow, it is also possible to cause ptosis of the upper eyelid by injecting Botox in the wrong anatomic location, or at the wrong depth, in the inferior portion of the forehead.  Botox injection of the forehead is an extremely useful tool to achieve subtle improvements in the upper third of the face of both men and women.  But, like any powerful tool, its indications, limitations, complications and side-effects must be well-understood by a physician who is experienced with the procedure, knows how to avoid the problems that can result, and is prepared to address any possible untoward effects that might arise

Hi there and thank you for the question.
Here are a few pointers: 
1. The dosage is variable depending on desired results and muscle strength. Your practitioner should provide you with information on this during consultation. 
2. On average units for forehead injection, around 10 units are required. 
Hope this helps and best wishes!
Dr Rupert