I must admit, I've never injected botox intending to "lower" the eyebrows. Usually, we try to stay high enough on the forehead to keep from dropping the brow. The closer you get to the brow, the higher risk of accidentally or, in your case, purposely dropping the brow. As you get really close to the brow, another potential side effect is to cause weakening of the muscle that lifts the eyelid. This can cause drooping of the eyelid, something known as ptosis. This side effect is relatively rare (has happened to me once out of hundreds and hundreds of injections) but that risk might a little higher if you are intending to drop the brow. If you had ptosis, it would effect how widely you can open your eyes.