With 1 mg = 1,000 mcg, start low and adjust based on tolerance to minimize side effects
Today, peptides--the majority of which are imported from China in bulk and later analyzed by third party labs to obtain a certificate of analysis (COA)--are largely sold as as "research chemicals usually as a powder that requires mixing with sterile water for subcutaneous injection, or as a nasal spray, often accompanied by pseudo-medical dosing guides, social media testimonials, and before-and-after photos
Allow the vial to reach room temperature before opening to prevent condensation inside the container
So, if you're feeling unwell due to a deficiency, restoring B12 levels might help you feel well enough to eat normally, but it's not "making" you gain weight in an unhealthy way
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Sikiric et al., 2020a))