Can botox in corrugator and procerus alone cause brow to drop?

Technically NO but practically YES in the wrong hands.  When injectors get too perfunctory during injection of the glabella (procerus and corrugators), the product can be misplaced and diffused to the levator muscle of the upper eyelid causing droopy upper lid or to a significant portion of the lower frontalis muscle causing a droopy brow.  However, if the injection is placed properly, one should actually see some medial brow elevation as the corrugator and procerus muscles are brow depressors.  Paralysis of these muscles should allow uninhibited pull of the frontalis muscles (a brow elevator).  You are correct in your request (anatomically speaking).  This stresses that only "core" injectors
should perform cosmetic injectables (facial
plastic surgeons, plastic surgeons, dermatologists, or occuloplastic
surgeons). This is because we all have a keen understanding of facial
anatomy and the effects of injecting Botox and fillers in different
regions of the face. Be sure you are seeing one of us with experience, and I wish you
well!

Typically an injection in the corrugator muscles will not cause the brow to drop. If Botox migrated to muscles that were not intended for Botox, you may experience a drop. Some patients report a heavy feeling in their brow for the first week or so after a Botox injection. This however is a feeling and not esthetically visible. All the Best. 

The corrugator and procerus muscles are brow depressors, meaning when they are active they pull the medial brow downward.  Treating these muscles with Botox will relax them and cause the medial brow to "lift".  Inadvertent treatment of the frontalis muscle instead of these muscles would cause the brows to droop.  

The corrugator muscle is a depressor of the brows.  Meaning that when it is contracted the brow will drop.  The frontalis is an elevator of the brow.  When contracted it raises the brow.  So Botox to the corrugator alone will not cause brow droop.  The brow will only droop if the botox is injected into the wrong muscle.

Botox injected into the middle brow can cause the brow to droop if some of the frontalis was effected.  In some people the frontalis is lower than you think and often can be mildly effected with only corrugator and procerus injection.  Especially if the lateral portion of the corrugator was injected and the botox diffuses up into the frontalis.  Nothing you can do now but wait 2-3 months for the botox to begin wearing off.  Best of luck, Dr. Emer.

In my experience with Botox, I would say treatment of the horizontal lines across the forehead is what normally can cause some brow drop.  This does not happen all of the time and it is temporary.  The closer the injection is to the brow, the more likely that there will be some dropping of the brow.  I do not believe there is any significant brow drop with only treatment of the corrugators and procerus, but this is dependent on the placement of injections. 

If only your corrugators and procures were treated it is doubtful that it could drop your lateral brow.  This could happen if botox was placed low on the lateral brow.

Typically brow drop occurs from Botox neurotoxin injections to close to certain portions of the eyebrow.

Botox is a fabulous procedure and the droop is not from the corrugator muscle. Perhaps what your doctor recommended was that if the injections were done too low you can get a lowered forehead and may appear as a droop. For the best cosmetic results please consult a board certified dermatologist who has experience with Botox. 

If placement of the Botox is correct, there should be no risk of brow ptosis. In fact, brow elevation should occur because the corregators depress the brow. I jetting the forehead is what will drop brows usually because the frontalis is the only brow elevator.