Vitamin D
Vitamin D Leohoho/Unsplash

Millions of people across the UK are being urged to take vitamin D during the darker months, but health experts are warning that a routine meant to boost wellbeing can quietly backfire. Subtle symptoms such as persistent thirst, stomach upset or mental fog may signal that a supplement widely viewed as harmless is being taken in excess, often without people realising.

An NHS spokesperson recently reminded the public that from October to March, sunlight in the UK is too weak for the body to make enough vitamin D naturally. As a result, adults are advised to take a daily supplement to help maintain bone, muscle and immune health. However, specialists say problems can arise when people combine multiple vitamin D sources or opt for high strength products.

What the NHS Actually Recommends

According to NHS England, the recommended daily intake for most adults is 10 micrograms, also known as 400 IU, during autumn and winter. This level is considered safe and effective for supporting bone health when sunlight exposure is limited.

Vitamin D plays a key role in calcium absorption, but too much can lead to raised calcium levels in the blood, a condition known as hypercalcaemia. This imbalance can affect digestion, hydration, muscles and even cognitive function.

How Overuse Happens Without People Noticing

Health experts say excess intake rarely comes from a single tablet. Instead, it often happens when people double up on vitamin D through multivitamins, sprays, gummies and fortified foods. Confusion between international units and micrograms can also make it difficult to track total intake.

Consumer group Which? has previously found vitamin D supplements on sale containing doses up to 12.5 times the recommended safe upper limit. With around 42% of UK adults reporting vitamin D use in the past year, the risk of accidental overuse is not insignificant.

Symptoms Experts Say Should Not Be Ignored

Biomedical scientist Tobias Mapulanga has warned that certain symptoms appearing soon after increasing vitamin D intake may be a red flag rather than a seasonal illness.

One of the earliest signs is frequent thirst accompanied by increased urination. This can occur when excess vitamin D disrupts fluid and salt balance, leaving people feeling constantly dehydrated.

Digestive symptoms are another warning. Nausea, persistent burping or stomach discomfort can develop, particularly when high dose supplements are taken on an empty stomach or in flavoured sprays and gummies that contain additional additives.

Muscle cramps and bone aches may also appear. While vitamin D is often taken to support musculoskeletal health, too much can interfere with mineral balance, increasing the likelihood of cramps, stiffness and discomfort.

Cognitive symptoms such as headaches and brain fog have also been linked to elevated calcium levels. Some people report difficulty concentrating or a sense of mental cloudiness that eases once intake is reduced.

Evidence of Real World Harm

NHS England has recorded dozens of incidents over a two-year period in which high strength vitamin D was administered more frequently than intended, with some cases requiring hospital treatment for hypercalcaemia.

These incidents were reported by Bristol Live, highlighting how dosing errors can occur even in clinical settings.

What to Do If Symptoms Appear

Health professionals advise returning to a single 10 microgram vitamin D supplement if symptoms develop and stopping any additional products containing vitamin D. Checking labels on cereals, drinks and multivitamins can help prevent accidental stacking.

Experts also recommend taking vitamin D with food, staying well-hydrated and seeking medical advice if symptoms persist. While vitamin D remains an important supplement for many, the message from health authorities is clear. More is not always better, and paying attention to warning signs can prevent a well intentioned health habit from doing more harm than good.

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