Vitamin B12 supplements only helps tiredness if you’re genuinely B12 deficient AND if your tiredness is due to that deficiency, as opposed to other reasons. If your levels are already normal, taking extra B12, whether as tablets or injections, hasn’t been shown to boost energy, despite how it’s often marketed to consumers. Persistent tiredness has many possible causes, so if you’re feeling constantly fatigued and your B12 is normal, it’s worth speaking to a doctor about what else might be going on rather than assuming a supplement will fix it. They may run a wider set of blood tests, discuss your sleep quality, mental health, medications you take and other health issues that could be contributing.