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Brain fog often has a cause worth checking first, such as poor sleep, menopause or low vitamin B12. Vitamin B12, B5, magnesium and zinc have authorised claims for normal mental performance or psychological function. Bacopa and lion's mane are traditionally used for memory, with mixed results in small trials.

Vitamin B12 rarely causes side effects. The NHS says adults need about 1.5 micrograms a day and that taking 2 mg or less a day from supplements is unlikely to cause harm. Rare skin reactions have been reported, mostly with high-dose injections. Some medicines, including metformin and acid-reducing tablets, can lower B12 levels.