NAD+ at pH 6 is in a stable range (as discussed earlier), where it’s less prone to hydrolysis. Vitamin B12, depending on its form, is also generally stable in a pH range of 4-7. For example, cyanocobalamin (a common injectable form of B12) is stable in slightly acidic to neutral conditions, with optimal stability around pH 4.5-7. So, at pH 6, both compounds should theoretically coexist without immediate degradation. B12 can be sensitive to light, heat, and certain chemical interactions. If the NAD+ solution has impurities or if the pH shifts after mixing (e.g., due to other additives like sodium bicarbonate, which I mentioned earlier), it could affect B12 stability. NAD+ is also light-sensitive, so the combined solution would need to be protected from light exposure.
For maximum stability, I would mix them just before use in same syringe, or inject them separately if you’re storing the solutions for longer than a week.