As winter settles in and daylight hours shrink, vitamin D supplements have become a near-daily ritual for millions of people. Pharmacies stock shelves with tablets, sprays, and gummies, while public health messaging repeatedly reminds people of the importance of maintaining vitamin D levels when sunlight is scarce. For many, taking vitamin D feels like a simple, responsible step toward staying healthy, supporting immunity, and protecting bones during the colder months. It is often viewed as harmless, routine, and unlikely to cause problems, especially when compared with prescription medication.

However, doctors and biomedical scientists are now warning that this widespread habit comes with an important caveat. While vitamin D is essential, taking too much or using high strength supplements can quietly trigger a range of symptoms that many people do not immediately connect to their daily vitamins. These warning signs are frequently mistaken for stress, dehydration, seasonal illness, or minor digestive issues. Health experts are now urging people to pay closer attention to how their bodies respond, particularly if new symptoms appear soon after increasing their vitamin D intake.

Why vitamin D is so heavily recommended in winter

Vitamin D is often referred to as the sunshine vitamin because the body produces it when skin is exposed to sunlight. In the UK, this natural process becomes difficult for much of the year due to low sun levels, especially between October and March. During this period, sunlight is not strong enough to trigger sufficient vitamin D production in the skin, making deficiency more likely.

Because of this, health authorities advise adults to consider taking a daily supplement throughout autumn and winter. An NHS spokesperson has previously explained: “From October to March we can’t make enough vitamin D from sunlight, so to keep bones and muscles healthy, it’s best to take a daily 10 microgram supplement of vitamin D. You can get vitamin D from most pharmacies and retailers.” This guidance has been widely shared and has encouraged many people to adopt supplementation as a seasonal routine.

Vitamin D plays a crucial role in helping the body absorb calcium, maintain strong bones, support muscle function, and contribute to immune health. These benefits are well established, which is why supplements are often seen as a low risk way to protect overall wellbeing. However, experts stress that recommended doses exist for a reason, and exceeding them can have unintended consequences.

When taking vitamin D starts to backfire

Unlike some vitamins that the body can easily flush out, vitamin D is fat soluble. This means that excess amounts are stored in the body rather than excreted through urine. Over time, this buildup can interfere with the body’s calcium balance, potentially leading to a condition known as hypercalcaemia, where calcium levels in the blood become too high.

Biomedical scientist Tobias Mapulanga has warned that many people unintentionally overshoot safe levels, particularly during winter. He explained: “As the NHS encourages vitamin D in winter, many people are reaching for bottles in the darker months and accidentally overshooting, then blaming the fallout on seasonal bugs.” Because symptoms can develop gradually, people may not immediately suspect their supplements are to blame.

Research has also raised concerns about the strength of some products available to consumers. Investigations have found vitamin D supplements on sale containing doses many times higher than the recommended daily amount. In some cases, NHS England has recorded incidents where high strength vitamin D was taken more frequently than intended, with certain individuals requiring hospital treatment as a result.

Symptom one nausea burping or an upset stomach

Digestive problems are among the most common early signs that vitamin D supplementation may not be suiting someone, and doctors say they are often the first clue that the body is reacting badly to an increased dose. People have reported feeling persistently nauseous, experiencing stomach pain or cramping, burping excessively throughout the day, or dealing with constipation and vomiting after taking their supplements. Because these symptoms are so common and can be linked to food, stress, or minor illness, many people do not immediately suspect their daily vitamin as the trigger.

Experts say these digestive symptoms can be more noticeable when people use vitamin D sprays or gummies rather than standard tablets. These products often contain sweeteners, flavourings, and additional ingredients that can irritate the stomach lining, especially when taken on an empty stomach. Tablets taken alongside a proper meal are generally considered gentler on digestion, particularly when people stick closely to the recommended dose rather than opting for high strength products.

Tobias Mapulanga noted that queasiness, nausea, and tummy discomfort appearing soon after increasing vitamin D intake can be a clear signal that the supplement is backfiring rather than helping. He has warned that many people continue taking supplements despite ongoing stomach issues, assuming the symptoms will pass. Doctors advise that if digestive problems persist, the safest option is to stop taking vitamin D and seek medical advice before resuming or changing products.

Symptom two constant thirst and frequent urination

Feeling unusually thirsty or needing to urinate more often than normal can easily be brushed off as dehydration, too much caffeine, or the result of a busy workday. Many people assume they simply need to drink more water or cut back on coffee. However, doctors warn that when these symptoms appear suddenly and persist, they can also point to excessive vitamin D intake.

According to medical experts, too much vitamin D can disrupt the body’s fluid and salt systems by increasing calcium levels in the blood. When this happens, the kidneys are forced to work harder to filter the excess calcium, which can result in constant thirst and repeated trips to the bathroom throughout the day and night. Over time, this strain can leave people feeling tired, dehydrated, and generally unwell.

Mapulanga highlighted this pattern, explaining that “when constant thirst and needing the loo more often” appear soon after increasing supplementation, it should not be ignored. These signs suggest that the body is struggling to maintain its normal balance, and continuing to take vitamin D despite these symptoms could increase the risk of more serious complications.

Symptom three headaches brain fog and trouble thinking clearly

Many people take vitamin D hoping it will boost energy levels, improve mood, or help them feel more alert during darker months. It is often associated with fighting fatigue and supporting mental wellbeing. Ironically, doctors say that taking too much vitamin D can sometimes produce the opposite effect, leading to headaches and difficulties with clear thinking.

People have reported experiencing brain fog, trouble concentrating, confusion, and persistent headaches after increasing their vitamin D dose. These symptoms can interfere with work and daily life, yet they are often blamed on stress, lack of sleep, or seasonal low mood. Because of this overlap, it can take time before people realise that supplements may be contributing to how they feel.

Mapulanga explained that “headaches or brain fog” appearing alongside other symptoms may indicate that vitamin D levels are too high. If mental clarity worsens after starting or increasing supplements, experts recommend reassessing the dose or stopping supplementation altogether until proper medical advice is sought.

Symptom four bone and muscle aches

Perhaps the most surprising warning sign linked to excess vitamin D is bone and muscle pain, especially since the supplement is commonly taken to support bone health. Doctors say that new or worsening aches that appear shortly after starting or increasing vitamin D should raise concerns rather than be ignored.

Some people experience muscle cramps, stiffness, weakness, or persistent bone discomfort that does not improve with rest or gentle exercise. In these cases, excess vitamin D may be interfering with calcium regulation instead of supporting it, leading to pain rather than protection. These symptoms can gradually worsen if supplementation continues unchecked.

Experts advise that if bone or muscle pain develops alongside other warning signs such as thirst, nausea, or headaches, people should stop taking vitamin D. Drinking plenty of water, eating foods rich in minerals, and seeking professional medical guidance are strongly recommended.

Expert advice and final warning

Health experts stress that vitamin D remains beneficial for many people when taken correctly and at appropriate doses. The key message is moderation and awareness, particularly when different products, strengths, or formats are combined. Taking multiple supplements that contain vitamin D or choosing high strength formulations without guidance can significantly increase the risk of side effects.

Summing up the risks, Tobias Mapulanga said: “The stakes are clear: the right amount supports you, but doubling up can turn a well meant routine into feeling worse rather than better.” His warning reflects growing concern among health professionals that well intentioned habits can quietly become harmful when limits are exceeded.

Doctors urge anyone experiencing these symptoms to stop supplementation and consult a healthcare professional. Vitamin D should enhance wellbeing rather than undermine it, and paying attention to early warning signs can help ensure that a simple winter supplement does not quietly turn into a serious health problem.

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