It depends. Oral vitamin B12, in the form of tablets and sublingual drops, is freely available at pharmacies and supermarkets, and is enough for many people whose deficiency is dietary. Injectable B12 (hydroxocobalamin) is a different matter: in Australia it’s technically a prescription-strength medicine, but in practice some pharmacies will sell the ampoules over the counter at the pharmacist’s discretion, sometimes in limited quantities, while others (including some larger chains) will ask for a prescription. Even where you can buy it, the injection needs to be given by a doctor or practice nurse. It’s important to have your levels checked and the cause looked into. A doctor can confirm whether you’re deficient, look for the underlying cause, and prescribe or arrange injections if you need them.