Can Taking Vitamin D Cause Tiredness? Understanding Deficiency vs. Toxicity

December 21, 2025 •

4 min read

Studies show that correcting vitamin D deficiency can significantly improve fatigue in many individuals, yet paradoxically, excessive intake can also lead to tiredness. This dual effect often causes confusion for those asking, "Can taking vitamin D cause tiredness?" This article explores the difference between fatigue caused by a lack of the vitamin and the very rare instances of tiredness resulting from an overdose.

Quick Summary

The relationship between vitamin D and energy levels is complex; while deficiency is a common cause of fatigue, a rare but dangerous overdose can also manifest as tiredness. This article clarifies how to differentiate between the symptoms of insufficient vitamin D versus excessive intake.

Key Points

Deficiency Causes Fatigue: Low vitamin D levels are a common cause of tiredness, exhaustion, and muscle weakness in many individuals.
Toxicity Causes Fatigue: Extremely high doses leading to vitamin D toxicity (hypercalcemia) can also cause tiredness, but this is a very rare and serious condition.
Rare Overdose Side Effect: Fatigue from over-supplementation is uncommon and is typically accompanied by other severe symptoms like nausea, confusion, and excessive urination.
Get Tested to Know: The only way to determine if your fatigue is related to a deficiency or a rare toxicity is to have a doctor check your blood vitamin D levels.
Practice Safe Supplementation: Stay within the recommended daily dosage of 600-800 IU for most adults, and never exceed 4,000 IU per day without strict medical supervision.
Sun Exposure is Not a Risk: It is impossible to get vitamin D toxicity from sun exposure alone, as the body regulates its own production.

The Dual-Sided Effect of Vitamin D on Energy

Vitamin D is a crucial nutrient, acting more like a hormone in the body than a traditional vitamin. It plays a vital role in regulating countless bodily functions, from calcium absorption for bone health to influencing immune response and mood. When levels are optimal, many people report feeling more energetic. However, when levels are out of balance, either too low or too high, it can disrupt these processes and lead to a variety of symptoms, including fatigue. The key difference lies in the prevalence and severity of the cause.

Why Vitamin D Deficiency Causes Fatigue

Vitamin D deficiency is a widespread issue and one of the most common nutritional causes of unexplained fatigue. When your body doesn't have enough vitamin D, several metabolic and physiological processes are affected, directly impacting your energy levels.

How Vitamin D Influences Energy

Impact on Muscles: Vitamin D receptors are present in muscle tissue, and a deficiency can lead to muscle weakness and myalgia (muscle pain), which can manifest as a general feeling of tiredness and reduced physical performance.
Calcium and Parathyroid Hormone: A severe vitamin D deficiency leads to poor calcium absorption. In response, the body increases parathyroid hormone production, which pulls calcium from the bones to maintain blood levels. This process can cause bone pain and further muscle-related symptoms, exacerbating fatigue.
Mitochondrial Function: Some research suggests that vitamin D may impact mitochondrial function, the cellular powerhouses that generate energy. Low vitamin D levels could impede these processes, resulting in a reduction of overall energy.
Inflammation and Neurotransmitters: Vitamin D helps modulate immune function and reduce inflammation. An imbalance can contribute to inflammatory processes linked to fatigue. The vitamin also influences the production of neurotransmitters like serotonin and dopamine, which regulate mood and energy.

Symptoms of vitamin D deficiency include:

Persistent, unexplained fatigue and tiredness
Bone and joint pain
Muscle weakness and cramps
Frequent illness or infections
Mood changes, including depression

When High Doses Can Make You Tired

It is extremely rare to experience tiredness from a vitamin D supplement unless you are taking a dangerously high dose over a prolonged period, leading to vitamin D toxicity, also known as hypervitaminosis D. The National Institutes of Health (NIH) states that symptoms of toxicity are most likely to appear with daily intakes exceeding 10,000 IU. This is distinct from the typical doses found in over-the-counter supplements.

The Danger of Hypercalcemia

The primary mechanism behind vitamin D toxicity is not the vitamin itself but the dangerously high levels of calcium it causes in the blood, a condition called hypercalcemia. Vitamin D's main function is to help the body absorb calcium, so an overdose causes excessive calcium absorption, which overwhelms the body's systems.

Symptoms of Vitamin D Toxicity

Neurological: Confusion, disorientation, dizziness, and irritability.
Renal: Increased thirst and frequent urination, which can lead to dehydration.
Gastrointestinal: Nausea, vomiting, loss of appetite, and constipation.
Cardiovascular: High blood pressure and irregular heartbeat.
Musculoskeletal: Muscle weakness and bone pain.

Deficiency vs. Toxicity: A Comparison


Symptom	Vitamin D Deficiency	Vitamin D Toxicity
Fatigue	Common, often subtle and nonspecific	Rare, usually accompanied by other severe symptoms
Nausea/Vomiting	Less common, not typically a primary symptom	One of the earliest signs, directly linked to high calcium
Thirst/Urination	Not directly related, though dehydration can contribute to fatigue	Excessive thirst and frequent urination are hallmark signs of hypercalcemia
Muscle Symptoms	Weakness, aches, and cramps are common	Can cause muscle weakness, twitches, and pain
Bone Health	Bone pain, soft bones (osteomalacia), and increased fracture risk	Bone pain and loss can occur as calcium is pulled from bones
Mental State	Can cause depression or mood changes	Confusion, disorientation, and irritability are common neurological signs

Safe Supplementation and How to Avoid Problems

The confusion around vitamin D and tiredness highlights the importance of proper medical guidance. Most people will never come close to experiencing toxicity from normal supplement use. Adhering to the following advice can help ensure your supplement routine is both safe and effective.

Get Tested: Before beginning or significantly changing a supplement regimen, have a healthcare provider check your blood serum 25-hydroxyvitamin D levels. This will establish a baseline and confirm if you are deficient.
Follow Recommendations: The NIH recommends a daily intake of 600–800 IU for most adults. Do not exceed the safe upper limit of 4,000 IU daily without a doctor's supervision. Some prescriptions for severe deficiency may involve higher doses, but this is always under strict medical monitoring.
Monitor Symptoms: If you begin taking a supplement and notice an increase in tiredness, accompanied by other unusual symptoms, stop taking the supplement and contact your doctor immediately. This could indicate an underlying issue or an improper dosage.

Conclusion

For the vast majority of people, taking a vitamin D supplement is far more likely to alleviate tiredness than to cause it. Fatigue is a classic and common symptom of vitamin D deficiency, a condition that affects many people worldwide. In contrast, fatigue from vitamin D toxicity is a very rare and serious side effect resulting from excessively high, unmonitored doses, not standard supplementation. By consulting with a healthcare professional, getting your levels tested, and following recommended dosages, you can safely and effectively use vitamin D to boost your energy, rather than having to worry about it being the cause of your fatigue.

For more comprehensive information on vitamin D toxicity, consult the National Institutes of Health's article on the subject.

Frequently Asked Questions

Symptoms of a vitamin D deficiency can include fatigue, bone and muscle pain, muscle weakness, cramps, mood changes, and frequent illness or infections.

Fatigue from vitamin D toxicity is usually severe and is accompanied by other symptoms like nausea, vomiting, loss of appetite, confusion, excessive thirst and urination, and muscle weakness.

No, it is not common to feel tired from a normal vitamin D supplement. In fact, for those who are deficient, supplements typically help resolve fatigue. Tiredness from toxicity is very rare.

Since fatigue can be a symptom of both deficiency and a rare toxicity, the only way to accurately diagnose the cause is through a blood test ordered by a healthcare provider.

The National Institutes of Health sets the safe upper limit for daily vitamin D intake at 4,000 IU for adults. For most adults, a typical recommended daily dose is 600-800 IU.

No, it is impossible to get vitamin D toxicity from sun exposure. The body has a built-in mechanism that regulates how much vitamin D it produces from sunlight, preventing an overdose.

If you experience increased fatigue or other unusual side effects after starting a vitamin D supplement, you should stop taking it and consult your doctor. They can determine if the supplement is the cause or if there is another underlying issue.