Brain fog: what supplements have evidence (and what to check first)

"Brain fog" is a colloquial term for difficulties with thinking [1]. A 2025 review of research across more than a dozen chronic conditions found the term used for a variable set of overlapping symptoms involving cognition, fatigue and mood, and defined variously as a distinct symptom, a syndrome or a nonspecific term [2]. Its authors argued that loose language, shared cognitive problems and non-cognitive factors explain the overlap better than one shared biology [2].

What people mean varies. Researchers who read 717 social media posts about brain fog found 141 first-person descriptions: forgetfulness (51), difficulty concentrating (43), dissociative feelings (34), mental slowness and excessive effort (26), trouble communicating (22), fuzziness or pressure (10) and fatigue (9) [1]. Half the posts sat in forums about illness, and 134 were in forums about using or stopping drugs [1]. A neurologist writing for other clinicians notes that patients use the term for a wide range of experiences, and that doctors are probably sometimes wrong about what a patient means by it [3].

As of November 11, 2026 we found no randomized, placebo-controlled trial in healthy adults that tested a supplement with brain fog as its primary outcome (PubMed and Europe PMC searches, November 11, 2026). So this page maps the experience to outcomes that trials do measure.

Table: What people describe as brain fog, and the outcome trials measure

The NHS gives advice on tiredness rather than on brain fog itself. It says that tiredness you cannot explain, which has gone on for a while without getting better, may be a sign of a medical condition [33]. It advises seeing a GP if you have been feeling tired for a few weeks and are not sure why, or if tiredness affects your daily life [33]. MedlinePlus gives the same advice after weeks of tiredness and notes that fatigue itself is not a disease but a symptom [34].

MedlinePlus lists anemia, infections, thyroid disease and sleep apnea among the medical causes of fatigue, and names some medicines, such as antidepressants, antihistamines and medicines for nausea and pain [34]. The NHS lists loud snoring and gasping, snorting or choking noises at night as signs of sleep apnea [33]. A clinician will likely do a physical exam and order lab tests [34], such as blood tests to check for anemia [33].

Low iron, even without anemia

The NHS lists tiredness and lack of energy among the symptoms of iron deficiency anemia, and a GP usually checks with a blood test [36]. Even without anemia, iron deficiency can be associated with fatigue [9]. A 2025 meta-analysis of 18 studies (12 randomized trials and 6 before-and-after studies) in 1,408 non-anemic participants, including 1,176 menstruating adults, found that in the randomized trials iron improved fatigue (d = 0.34) and short-term memory (d = 0.53) but not attention [9]. The effects were absent once participants with iron deficiency were excluded, and the authors concluded that iron deficiency may affect brain function before anemia emerges, potentially requiring identification and treatment [9]. High-dose iron can cause nausea and constipation, and FDA requires a label warning that accidental overdose of iron-containing products is a leading cause of fatal poisoning in children under 6 [38].

An older meta-analysis of 14 randomized trials in children aged 6 and over, adolescents and women found that iron improved attention and concentration irrespective of baseline iron status (SMD 0.59) [10]. Evidence is mixed: the older trials were generally small, short and methodologically weak, with signs of modest publication bias [10], while the newer analysis found no attention effect in non-anemic people [9].

Vitamin B12

The NHS lists problems with memory, understanding and judgment, and feeling weak or tired, among the symptoms of vitamin B12 or folate deficiency anemia, and says some of them can occur in people who are deficient but have not developed anemia [35]. It notes that some problems caused by the condition can be irreversible if left untreated [35]. A 2026 expert review describes diverse signs of B12 deficiency, including hematological, neurological, psychiatric and thromboembolic ones, and notes there is still no universally accepted definition of deficiency [11].

Deficiency can come from too little B12 in the diet, from impaired absorption, or as a side effect of medicines [11]. In six randomized trials in people taking metformin for type 2 diabetes or polycystic ovary syndrome (clinical populations), serum B12 was lower with metformin than with placebo or rosiglitazone (mean difference -53.93 pmol/L) [12]. In a case-control study comparing 25,956 people with newly diagnosed B12 deficiency with 184,199 people without it, a supply of proton pump inhibitors lasting 2 years or more was associated with higher odds of deficiency (OR 1.65) [13]. If you take either kind of medicine, your doctor or pharmacist can tell you whether a B12 check makes sense.

Thyroid

A narrative review reports that some people treated for an underactive thyroid describe brain fog; symptoms vary but commonly include fatigue, low mood and difficulties with memory and executive function, and often predate the diagnosis [14]. The same review notes that thyroid-related brain fog has not been well defined or measured [14]. These findings come from a clinical population, and results may not apply to healthy adults.

Several free factors move the same test scores that supplements are judged on.

Sleep comes first. A meta-analysis of 70 articles covering 147 cognitive tests found that the effects of short-term total sleep deprivation ranged from small and non-significant for reasoning accuracy to large for lapses in simple attention (g = -0.776) [4]. The NHS suggests keeping the same sleep times and aiming for 6 to 9 hours [33]. More on this goal is on our sleep hub.

Fluid matters too. A meta-analysis of 33 studies in 413 people who were dehydrated by 1% to 6% of body mass found a small but significant impairment in cognitive performance overall (ES = -0.21), with larger effects on attention tasks (ES = -0.52) [5]. Impairment was greater in studies where people lost more than 2% of body mass (ES = -0.28) [5].

Movement points the other way. A 2025 meta-review of 30 systematic reviews with meta-analyses (383 studies, 18,347 participants) found that a single bout of exercise improved cognitive function with a small-to-medium effect (mean SMD = 0.33) in studies from children to older adults, including on attention tasks (mean SMD = 0.37) [6].

Caffeine habits can cause fog as well as relieve it [7][15]. A review of 57 experimental and 9 survey studies found 10 validated withdrawal symptoms, including fatigue, decreased alertness, difficulty concentrating and feeling "foggy/not clearheaded" [7]. Symptoms typically began 12-24 hours after stopping, peaked at 20-51 hours and lasted 2-9 days, and stopping doses as low as 100 mg a day produced them [7]. Caffeine also costs sleep: in a meta-analysis of 24 studies it reduced total sleep time by 45 minutes and lengthened the time to fall asleep by 9 minutes [8]. The authors estimated that coffee (107 mg per 250 mL) would need to be drunk at least 8.8 hours before bedtime to avoid a reduction in total sleep time [8].

For difficulty concentrating, the closest measured outcome is attention. A 2025 meta-analysis of 31 randomized, double-blind, placebo-controlled trials in 1,455 rested healthy adults found that caffeine improved accuracy (g = 0.27) and reaction time (g = 0.28) on attention tests [15]. In trials using 200 mg or more, both improved more than at lower doses, but accuracy improved only up to a point and then declined as doses rose further [15].

In people short on sleep, a meta-analysis of 45 publications found caffeine improved accuracy on attention tests (g = 0.68) and executive function (g = 0.35) [16]. The main cautions are the sleep cost and withdrawal effects described above [7][8]. Safety, interactions and legal limits are on the caffeine monograph, and other options for concentration are on our focus hub.

A meta-analysis of 11 randomized placebo-controlled studies in healthy volunteers found moderate effect sizes favoring caffeine combined with L-theanine for self-rated alertness and attention-switching accuracy in the first 2 hours after a dose [17]. The authors saw trends suggesting caffeine dose mattered more than L-theanine dose [17]. The grade above therefore applies to L-theanine with caffeine, not on its own. Details are on the L-theanine monograph.

The creatine evidence relevant here comes from sleep-loss studies [18]. A 2026 systematic review found only five studies of creatine during acute sleep deprivation and reported a positive trend that may vary by cognitive domain, but concluded that research is sparse [18]. In one placebo-controlled trial in healthy adults, a single dose of 0.35 g/kg taken during 21 hours without sleep improved cognitive performance and processing speed [19]. A follow-up trial in 29 healthy adults used 0.2 g/kg and found less decline on logic, number, processing-speed and vigilance tasks, with a smaller effect than the higher dose [20]. Both trials came from the same research group [19][20]. Doses used in longer studies and safety are on the creatine monograph.

Rhodiola rosea has been studied for physical and mental fatigue in healthy and clinical groups [21]. A systematic review found 11 controlled trials; three of five randomized trials of mental fatigue reported rhodiola effective, but every included study had either a high risk of bias or reporting flaws [21]. The reviewers concluded that research on rhodiola's efficacy is contradictory [21]. The rhodiola monograph covers later trials, on which its grade mainly rests, and the safety flag.

For ginseng, the evidence is mixed, and the analyses pooled different populations [22][23]. A 2016 meta-analysis of 12 randomized trials (630 participants) found less fatigue with ginseng than placebo in a pooled analysis of four trials (SMD = 0.34), but its authors judged the evidence insufficient [22]. A 2023 meta-analysis of 19 randomized trials of ginseng and ginseng herbal formulas, in people with general, chronic or disease-related fatigue, found no significant overall reduction in fatigue severity (SMD -0.36, 95% CI -0.82 to 0.11) [23]. In its subgroup of general, non-disease-specific fatigue, ginseng did better than controls (SMD -0.48), and the authors described the effect sizes as small [23]. Interactions and the safety flag are on the Panax ginseng monograph, and other fatigue options are on the energy hub.

The NHS says many symptoms of B12 or folate deficiency anemia improve with treatment, and that a blood test helps diagnose it [35]. In people without overt B12 deficiency or advanced neurological disorders, a meta-analysis of 16 randomized trials with 6,276 participants found no evidence that B12 alone or a B complex affected any area of cognitive function [24]. Only one of those trials reported fatigue, so fatigue could not be analyzed [24]. A Cochrane review of 14 trials (27,882 participants), most of them people over 60 with a history of heart or blood-vessel disease, found that B vitamin supplements in cognitively healthy adults probably have little or no effect on global cognitive function at any time up to 5 years [25]. The B vitamins monograph covers upper limits and the medicines that interact with them.

• Omega-3 for long-term cognition. A meta-analysis of 38 randomized trials (49,757 participants, mostly older adults) found that long-chain omega-3 probably has little or no effect on new cognitive impairment or other neurocognitive outcomes [26]. N, no benefit shown See the omega-3 monograph.
• Ginkgo for focus. A meta-analysis of randomized trials in healthy people found an attention effect of d = -0.08 across 910 participants, non-significant and close to zero [27]. N, no benefit shown See the ginkgo monograph.
• Multi-ingredient "brain fog" kits. Apart from post-illness research, the only supplement study we found that recruited people for self-reported brain fog was an open-label, single-arm 8-week study of 39 people whose fog was attributed to trauma, disease or chemotherapy, with no control group [28]. It was funded by Griggs Holding, LLC, which paid three of the four authors consulting fees for the project [28]. The authors state that practice and placebo effects cannot be ruled out and that their findings are hypothesis-generating [28].

Much of the research on brain fog comes from people with an illness, and those findings describe clinical populations; results may not apply to healthy adults. In a meta-analysis of studies 12 or more weeks after a COVID-19 diagnosis, the pooled proportion with fatigue was 0.32 (25,268 people) and with cognitive impairment 0.22 (13,232 people) [30]. In a 2025 meta-analysis, people with brain fog after COVID-19 had lower cognitive performance (g = -0.63) and higher fatigue (g = 2.64) than controls, but assessment tools varied widely and heterogeneity was high (I2>70%) [29].

Menopause studies point to a temporary change. In the Study of Women's Health Across the Nation, 2,362 women were followed for 4 years [31]. Processing-speed scores of late perimenopausal women did not improve with repeated testing, and delayed recall did not improve during early or late perimenopause; improvement rebounded after menopause, which the authors read as a sign that these difficulties may be time-limited [31]. A 2025 systematic review of 24 studies (5,629 participants), with a meta-analysis of six, found only a small correlation between women's own sense of their cognition and measured learning efficiency (r = .12), with no significant correlation in other domains [32].

In the US, under DSHEA, FDA does not have the authority to approve dietary supplements before they are marketed, and it generally does not approve dietary supplement claims or other labeling before use [37]. A firm making a structure/function claim must have substantiation that the claim is truthful and not misleading [37]. Status in the UK, Canada and Australia is covered on each compound's monograph, and our guide Are nootropics safe? covers quality and safety across the category.

Each badge is a grade for one compound and one outcome, based on human trials and updated when new studies are graded. No compound on this site has a grade for "brain fog" itself, because trials measure attention, fatigue and memory instead. The evidence matrix lists every grade, and how we grade evidence explains the letters. This page is educational guidance, not medical advice; see our disclaimer.

What is the best supplement for brain fog?

As of November 11, 2026 we found no randomized, placebo-controlled trial in healthy adults that tested a supplement with brain fog as its primary outcome (PubMed and Europe PMC searches, November 11, 2026). For the closest measured outcome, attention, caffeine improved accuracy and reaction time in rested healthy adults in 31 placebo-controlled trials [15].

Can low iron or vitamin B12 feel like brain fog?

The NHS lists problems with memory, understanding and judgment among the symptoms of vitamin B12 or folate deficiency anemia [35], and tiredness and lack of energy among the symptoms of iron deficiency anemia [36]. In a meta-analysis of non-anemic children, adolescents and menstruating adults, iron's effects on fatigue were absent once participants with iron deficiency were excluded [9]. High-dose iron can upset the stomach, and accidental overdose is a leading cause of fatal poisoning in children under 6 [38].

Does caffeine help or cause brain fog?

It can do both. Caffeine improved attention in rested healthy adults in placebo-controlled trials [15], but feeling foggy or not clearheaded is one of the validated symptoms of caffeine withdrawal, which appeared after stopping doses as low as 100 mg a day [7]. Caffeine also shortened total sleep by 45 minutes in a meta-analysis of 24 studies [8].

Is brain fog after an illness or during menopause the same as everyday brain fog?

Not necessarily. Across conditions, brain fog has been defined as a distinct symptom, a syndrome or a nonspecific term [2]. In a cohort of 2,362 midlife women, perimenopause was linked to women not learning as well on repeated tests as before, with a rebound after menopause [31]. Studies after COVID-19 describe a clinical population with lower cognitive performance and higher fatigue than controls [29], so their findings may not apply to healthy adults.

When should I see a doctor about brain fog?

Neither source gives advice on brain fog itself; for tiredness, the NHS advises seeing a GP if you have felt tired for a few weeks and are not sure why, or if tiredness affects your daily life [33]. MedlinePlus advises contacting a health care provider after weeks of tiredness; they will likely do a physical exam and order lab tests [34].