A plant-based diet is good for most things, but a handful of nutrients are genuinely harder to get enough of, and those are the ones worth supplementing deliberately rather than guessing at. The list is short. Getting these few right matters more than a cupboard full of greens powders.
Vitamin B12: the non-negotiable one
Vitamin B12 is the clearest case. It is essentially absent from plant foods, so anyone eating fully plant-based needs a reliable source, whether a supplement or genuinely fortified foods. This is not optional or "nice to have": long-term low B12 causes fatigue and, eventually, nerve damage that can be permanent. A regular B12 supplement is the simplest insurance, and it is the single most important item on this list.
Iron: there, but harder to absorb
Plant foods do contain iron, but in the non-haem form that the body absorbs less efficiently than the iron in meat. You can close a lot of that gap by pairing iron-rich plant foods with vitamin C, which markedly improves absorption, and by keeping tea and coffee away from iron-rich meals. Whether you need a supplement depends on your ferritin, so a blood test is the honest way to decide rather than supplementing on principle.
Omega-3: from algae, not fish
The beneficial long-chain omega-3 fats EPA and DHA come mainly from oily fish. The body makes only a little from the plant form in flax and walnuts. An algal oil supplement gives you EPA and DHA directly from the same source the fish get it from, which is the cleanest way to cover this on a plant-based diet.
Vitamin D and a couple of others
Most people in the UK are short of vitamin D in winter regardless of diet, and a lichen-derived D3 gives vegans the better-absorbed form without an animal source. Iodine (if you avoid dairy, fish and iodised salt) and zinc (less available from plant foods) are worth being aware of, though they matter less than the first three.
If tiredness is your main reason for looking, the energy guide covers where to look first, and B12 and iron are usually the answer. To check any supplement against a medication you take, the free checker will show you the result and the reasoning.