Can Botox be injected beneath the nose to shorten a long philtrum?

That would likely lengthen your upper lip, NOT shorten it.  A lip shortening procedure can be done but that involves surgical excision.

If you inject Botox beneath the nose it serves to relax the muscle and give the appearance of lengthening the area, not shortening. Botox injected on or near the vermilion border can cause your upper lip to turn up. 

I have heard and tried many different techniques using Botox, but this is a new one for me.Physiologically, it does not make any sense, and, if anything, they should lengthen the area.Your friend's doctor either knows something that I and many others in the medical community don't, or they are creating something out of thin air.Best of luck,Mats Hagstrom, M.D.

Botox under the nose will weaken lip elevators and make the upper lip appear longer when smiling.  The lip may be shortened but requires a lip lift with an incision under the nostrils.  Good luck.  Donald R. Nunn MD  Atlanta Plastic Surgeon

 There are several muscles under the nose that can shorten the philtrum when they contract. These can  become a problem after a rhinoplasty, as they can also pull the nasal tip down. Injecting Botox (or other botulinum toxin) into any of these will have the opposite effect of what you seek; the philtrum will lengthen. There is, however, a simple surgical procedure that can be done in the office under local anesthesia to shorten the philtrum. The scar is placed just under the nasal sill and hides exceptionally well since it is totally in a crease. Without firsthand knowledge of exactly what was done and seeing the before and after, I would not trust what your friend had as being Botox injection. Make sure whomever you see is a board certified plastic surgeon and specializes in facial rejuvenation.