standard b12 injection dose Optimal Vitamin Dosage and Treating Deficiency B12 Injection Dosage and Frequency:

2009 Sep;139(9):1744-50

Vitamin B12 is most seen in oral supplementation or vitamins you can buy in the grocery store or local pharmacy

Biotin injections have not been studied in clinical trials specifically for cosmetic hair/nail improvement

I normally treat these issues with T3 thyroid hormone (to bypass thyroid conversion) but it would be highly likely that levothyroxine injections could do a better job by directly injecting small amounts of thyroid hormone into sore muscles

Vitamin B12, also known as cobalamin, is a water-soluble vitamin essential for various bodily functions

(2020) Comparison of sublingual and intramuscular administration of vitamin B12 for the treatment of vitamin B12 deficiency in children