If irritation develops, consult a physician
This is one reason why GHK-CU is frequently discussed in anti-aging and longevity-related research
Vitamin B12 is involved in Energy metabolism, DNA synthesis, red blood cell production, nervous system function, and reproduction/fertility
For emerging brands, that can be very useful because it allows them to build an entire brand ecosystem with one manufacturing partner rather than coordinating with several specialized suppliers
If it's due to diet, you can eat more red meat, chicken, fish, and dairy
Rotate the site every time, because injecting the same spot repeatedly can cause a fatty lump (lipohypertrophy) that slows absorption, a point the NHS is clear about when it advises you to rotate sites daily