Is there such thing as legal butt injections? Like a shot to make your butt bigger instead of implants?

Thanks for your question. 
Use of Sculptra in the buttocks is a recent trend which uses an FDA approved facial filler in a different location that it was approved. This is called off label use and the technique works best on thinner people who do not have enough fat for fat injections. See an expert. The injections are done in office under local anesthesia and you can add PRP for improvement in volume. There’s no downtime and you can resume exercise in 24 hours. There’s very little pain and you will be required to massage the area for several days. Sculptra grows slowly over 1 to 2 months. 

Sculptra can be used to enhance the buttocks. The buttocks need numerous vials of Sculptra over time to build volume, and in some instances the results are subtle and will need to be combined with fat or implants to achieve desired results. I recommend seeing a body shaping expert for a formal evaluation. Best, Dr. Emer.

Thank you for your question. There are currently no injections specifically approved by the FDA for the augmentation of the buttocks. At the discretion of a physician, some products can be used "off label" for buttocks augmentation. The most commonly used is Sculptra Aesthetic. Each vial of Sculptra will generate approximately 5 cc of volume (1 teaspoon) so it can take quite a few vials to achieve the desired results. This is best used in patients who do not have fat available to transfer. Most patients require 20 to 40 or more vials injected over 2 to 3 sessions to achieve the desired results.I hope this was helpful - Dr. Moore.

There are some surgeons injecting various types of fillers into the buttock. I co-authored an article on MedScape with one my mentors about safety measures related to buttock injections. Hope this helps.
-Raja Mohan MD