I am no expert on any of this but read this forum every day
[1] Adiposity and Microbiome Studies Paired with a low-fat diet, 5-amino-1-methylquinolinium was used to study whole-body adiposity and weight change in DIO mice, moving several measures toward age-matched lean controls within the study window

smaller droplets may reach the lungs Head position: Supine or reflex head extension during administration improves olfactory region deposition Volume control: Volumes exceeding 10 L per nostril in mice risk drainage into the trachea/lungs rather than CNS Systemic spillover: IN administration typically results in only 110% systemic bioavailability, so it is unsuitable when peripheral effects are the primary endpoint Practical Guidance for Route Selection Administration Schedule Considerations Dosing frequency should be matched to both the peptides half-life and the chosen route: Once-weekly peptides (semaglutide, tirzepatide, retatrutide, CJC-1295 DAC): SC only the slow-release profile requires subcutaneous deposition Daily peptides (BPC-157, GHK-Cu, TB-500): SC or IP, depending on whether local or systemic effects are desired Multiple daily peptides (ipamorelin, selank): SC or IN choose the least stressful route that achieves target bioavailability Short half-life CNS peptides (DSIP, Semax): IN preferred to avoid rapid systemic clearance Research Safety & Ethical Considerations Responsible peptide research requires rigorous attention to safety protocols, humane treatment of animal subjects, and experimental design that minimizes bias

They help reduce body weight by reducing hunger, slowing digestion, boosting metabolism, and improving fat breakdown (lipolysis) while preserving muscle mass
There are two main ways to inject B12 into the muscle (intramuscular) or just under the skin (subcutaneous)