Mental fog: what we know, and where CBD sits in the debate

The term entered everyday vocabulary via long Covid, where it denotes persistent cognitive difficulties. Outside that context, it has no official medical definition in France. The symptoms grouped under it are, however, entirely real and documented: difficulty concentrating, slowed reasoning, working-memory problems, a feeling of being bogged down, and fatigue out of all proportion to the effort made.

Inserm and several European reviews identify recognised causes for this type of complaint: chronic lack of sleep, prolonged stress disrupting the hippocampus via cortisol, deficiencies in iron, vitamin B12 or D, even mild dehydration, hormonal imbalances, adverse drug effects (sedating antihistamines, benzodiazepines), and digital cognitive overload linked to notifications and multitasking. Fog persisting for more than a few weeks, or accompanied by weight loss or severe sleep disturbance, warrants a medical consultation.

Before considering any supplement, the French agencies converge on lifestyle levers. Aiming for 7 to 9 hours of sleep at regular times comes first, followed by physical activity of at least 30 minutes five times a week, according to Santé publique France. Limiting screens, particularly in the hour before bedtime, and taking care over diet (omega-3s, B vitamins, adequate hydration) complete this foundation. If the complaint persists, first-line blood tests (full blood count, ferritin, TSH, vitamin D, B12) help rule out an organic cause.

The international literature on CBD and cognitive function remains preliminary and heterogeneous. A few underpowered trials suggest an acute anxiolytic effect at doses of 300 to 600 mg, far above common well-being doses; the Shannon 2019 study, published in The Permanente Journal, observed a self-reported improvement in sleep among anxious patients, but without a randomised control group. To date, no quality study demonstrates a direct effect of CBD on memory or concentration in healthy adults. EFSA considers these data insufficient to support a health claim, and none is entered in the European register for cannabidiol.

On 9 February 2026 EFSA published a provisional safe daily dose of 0.0275 mg/kg/day, roughly 2 mg/day for 70 kg, with an uncertainty factor of 400. Since June 2025 ANSES has been proposing a 1B reprotoxic classification for CBD at European level, and since 15 May 2026 ingested CBD products without Novel Food authorisation have been withdrawn from the market. CBD may not be presented as a treatment; it is not advised for pregnant or breastfeeding women, for minors or for people undergoing treatment, because of possible interactions via the cytochrome P450 enzymes. Our article on CBD's interactions with the liver sets out this mechanism in detail.

CBD is not a medicine. In the event of persistent cognitive difficulties, consult a GP or a neurologist.

Is mental fog an illness?

No, not in France. It is a functional complaint that may reflect a lack of sleep, chronic stress, a deficiency or a post-infectious syndrome. A consultation is called for if it persists.

Does CBD improve concentration?

No quality clinical study demonstrates this in healthy adults. The available literature remains preliminary and insufficient to support a recommendation.

Which tests should I ask my doctor for?

Full blood count, ferritin, TSH, vitamin D and B12 are the first-line tests for persistent mental fog. The doctor may add others depending on the clinical context.

How long does it take to recover good concentration?

Four to six weeks of sleep hygiene and stress reduction are generally needed before a clear improvement is seen. Consistency matters more than intensity.