If you take metformin for type 2 diabetes or PCOS, there is one supplement-related fact worth knowing that often goes unmentioned: long-term metformin lowers vitamin B12. It is the clearest example of a medicine quietly depleting a nutrient, and it is easy to catch and easy to fix once you know to look.
Why B12 drops
Metformin interferes with how the gut absorbs vitamin B12. After a year or more of daily use, B12 levels fall in a meaningful share of people, with estimates commonly in the range of one in ten to one in three. The symptoms of low B12 (fatigue, tingling in the hands or feet, low mood, brain fog) overlap with so many other things that it often gets missed, or blamed on the diabetes itself.
The fix is straightforward. An annual B12 check is sensible if you have been on metformin a while, and if it is low, a B12 supplement or a B-complex corrects it without needing to stop the metformin. This is one of the more useful things to flag to your GP, because it is a simple test that occasionally explains a symptom people have been carrying for months. It is also a good example of when a blood test genuinely changes what you should do.
Supplements that lower blood sugar too
The other thing to know about metformin is that several supplements lower blood sugar in their own right, and they stack with what metformin is already doing. Berberine, alpha-lipoic acid, inositol, chromium and bitter melon all have evidence here. None of them is off-limits, but adding one without watching your glucose can push you into a hypoglycaemic dip, especially in the first weeks. If you want to try one, home glucose monitoring at the start is the sensible safeguard, and worth mentioning to whoever manages your diabetes.
The simple version
Get B12 checked once a year on long-term metformin, and top it up if it is low. Treat glucose-lowering supplements as additive, not free, and monitor when you start one. To check a specific supplement against metformin, the free checker will show you the call and the reasoning. And if fatigue is the thing driving your interest in supplements, the first place to look is usually iron or B12 rather than a stimulant.