Why do I feel more tired after taking vitamins?

November 14, 2025 •

4 min read

According to a 2022 national survey, over 13% of US adults experience chronic fatigue, and for some, the problem appears after starting a vitamin regimen. This seemingly paradoxical effect of feeling more tired after taking vitamins has several surprising and common explanations beyond simple deficiency.

Quick Summary

Taking vitamins can sometimes lead to fatigue due to factors like excessive dosage, nutrient imbalances, or consumption on an empty stomach. Certain vitamins, including B vitamins and Vitamin D, can affect sleep cycles in high doses, while interactions with other medications can also play a role. Addressing improper timing, dosage, and product quality is crucial to mitigate these effects.

Key Points

Overdose Can Cause Fatigue: Excessive intake of fat-soluble vitamins (A, D, E, K) leads to toxic buildup, and even high doses of water-soluble vitamins can cause side effects like fatigue.
Empty Stomach Troubles: Taking supplements, especially minerals like iron and acidic vitamins like C, on an empty stomach can lead to nausea and discomfort, which is often perceived as fatigue.
Nutrient Imbalances are a Risk: High doses of one nutrient can compete with or mask the deficiency of another, leading to unexpected energy dips. For example, high calcium can interfere with iron absorption.
Timing Matters for B Vitamins: While B vitamins are crucial for energy metabolism, some—particularly B12 in high doses—can influence sleep cycles and may be best taken earlier in the day.
Consider Underlying Health Factors: Fatigue is a non-specific symptom with many potential causes. It's crucial to rule out other medical conditions or lifestyle factors before attributing tiredness solely to your supplements.
Product Quality Impacts Effects: Cheap supplements may contain inactive fillers or binders that can cause sensitivities and fatigue. Choosing a high-quality, third-party tested brand is essential.
Food First, Supplement Second: The best way to get nutrients is from a balanced diet. Supplements are meant to fill gaps, not replace whole foods, and should be used judiciously under medical guidance.

Unexpected Causes of Post-Vitamin Fatigue

When you start a new vitamin routine, you expect to feel revitalized, not drained. However, several factors can cause the opposite effect, leaving you feeling unexpectedly tired or drowsy. These range from simple timing issues to more complex physiological reactions to your supplement.

1. The Dangers of Vitamin Overload

Contrary to the belief that more is better, exceeding the recommended daily allowance (RDA) for certain vitamins and minerals can cause a toxic buildup in your system, known as hypervitaminosis.

Fat-soluble vitamins: Vitamins A, D, E, and K are stored in your body's fat tissues. Over time, excessive intake can lead to toxicity with symptoms like fatigue, nausea, and headaches. For instance, too much Vitamin D can cause hypercalcemia (excess calcium in the blood), which results in weakness and vomiting.
Water-soluble vitamins: While the body typically excretes excess water-soluble vitamins (B and C) through urine, megadoses can still cause problems. High doses of Vitamin B3 (niacin) can cause fatigue, and too much Vitamin C can lead to gastrointestinal upset that saps energy.

2. Taking Supplements on an Empty Stomach

Proper absorption can significantly affect how your body responds to a supplement. Taking certain vitamins on an empty stomach is a common mistake that can trigger fatigue and digestive issues.

Irritation and nausea: High-acid vitamins like Vitamin C and minerals like iron can irritate the stomach lining, causing nausea and discomfort that can feel like fatigue. Taking them with food acts as a buffer.
Poor absorption: Fat-soluble vitamins (A, D, E, K) require dietary fat for optimal absorption. Taking these on an empty stomach means they won't be absorbed effectively, and you won't get the intended benefits. Multivitamins often contain a mix of both fat- and water-soluble vitamins, making it best to take them with a meal.

3. Nutrient Imbalances and Interactions

Another subtle but impactful reason for post-supplement fatigue is the imbalance caused by competing nutrients. When you take high doses of one mineral, it can interfere with the absorption of another.

Calcium vs. Iron: Taking calcium and iron supplements at the same time can compromise the absorption of both minerals. If you are taking an iron supplement to combat fatigue from anemia, combining it with a high-calcium food or supplement could hinder its effectiveness.
Folic Acid vs. Vitamin B12: High doses of folic acid can mask the symptoms of a Vitamin B12 deficiency in older adults, potentially delaying treatment for nerve damage.

4. Quality and Inactive Ingredients

Not all supplements are created equal. The binders, fillers, and additives used to manufacture tablets can cause sensitivities or allergic reactions in some individuals, with fatigue being a possible side effect. Additionally, poor manufacturing processes may mean the supplement contains little or none of the advertised active ingredients.

Multivitamins vs. Single Supplements: A Comparison


Feature	Multivitamin	Single Supplement
Composition	Contains a broad spectrum of vitamins and minerals.	Contains a single vitamin or mineral, e.g., Vitamin D.
Dosage Control	Less precise; risk of over- or under-dosing specific nutrients.	Highly precise; allows for targeted correction of specific deficiencies.
Best For	General health support for those with balanced diets.	Individuals with diagnosed deficiencies or specific health needs.
Side Effect Risk	Higher risk of nutrient imbalance or interaction due to complex formulation.	Lower risk of interaction but higher risk of overdose if not monitored.
Fatigue Factor	May contain competing nutrients or heavy metals that contribute to fatigue.	Can cause fatigue if taken improperly (e.g., iron on empty stomach).

How to Avoid Feeling Tired After Taking Vitamins

To ensure your vitamin routine boosts your energy instead of sapping it, follow these guidelines:

Consult a professional: Before starting any supplement, talk to a doctor or registered dietitian to assess your needs and identify any underlying deficiencies.
Read the label: Always follow the recommended dosage and check for any warnings about taking with or without food. Choose reputable brands with third-party verification.
Take with food: To prevent nausea and improve absorption of fat-soluble vitamins, take your multivitamin with a meal containing some fat.
Time it right: If your multivitamin contains B vitamins, taking it in the morning can support energy production throughout the day. Avoid taking supplements with known sleep-influencing effects, like high-dose Vitamin B12, too close to bedtime.
Address underlying causes: Remember that persistent fatigue is a symptom with many possible causes, not just vitamins. A supplement should not be used as a cure-all without addressing other lifestyle factors such as diet, sleep, and exercise.

Conclusion

Feeling more tired after taking vitamins is a clear sign that something is amiss with your supplement regimen. Whether it's a subtle side effect of over-supplementation, an issue with timing, or an imbalance of nutrients, identifying the root cause is the key to resolving it. Instead of blindly increasing your dose, focus on smart supplementation, proper timing, and prioritizing a nutrient-rich whole-foods diet to truly boost your energy and well-being. Always consult a healthcare provider for personalized advice to ensure your supplements are working for, not against, you.

For more information on the intricate relationship between diet and energy, consult the National Institutes of Health's extensive research on vitamins and minerals: https://pmc.ncbi.nlm.nih.gov/articles/PMC7019700/

Frequently Asked Questions

Yes, taking excessive amounts of vitamins, particularly fat-soluble ones (A, D, E, K), can lead to a toxic buildup (hypervitaminosis) that causes side effects like fatigue and nausea. Even large doses of water-soluble vitamins can cause gastrointestinal issues that lead to fatigue.

Multivitamins can cause tiredness for several reasons, including improper timing, nutrient imbalances, or sensitivities to inactive ingredients. Taking them on an empty stomach or at the wrong time of day can disrupt digestion and energy levels.

Generally, it's best to take vitamins with a meal to improve absorption and reduce the risk of stomach upset. B vitamins, which aid energy metabolism, are often recommended in the morning. Fat-soluble vitamins require a meal with some fat for proper absorption.

While B12 is known for boosting energy, some people may feel drowsy, especially with high doses. This can be due to its role in melatonin production and regulating the sleep-wake cycle. Taking B12 in the morning can help align it with your body's energy needs.

To determine if vitamins are the cause, first evaluate your dosage, timing, and diet. If you suspect a connection, stop taking the supplement for a few days to see if your energy improves. For persistent fatigue, consult a healthcare provider to rule out underlying medical conditions.

Symptoms of vitamin toxicity can vary but often include fatigue, nausea, headaches, dizziness, and stomach upset. Signs become more pronounced with prolonged excessive intake, especially with fat-soluble vitamins.

It is always best to obtain nutrients from a balanced, whole-foods diet first. Supplements are meant to fill nutritional gaps, not replace food. A diverse diet ensures you get a complex array of nutrients that work synergistically.