What are the potential side effects and contraindications of vitamin D replacement therapy?

Potential side effects of vitamin D replacement therapy include hypercalcaemia, which can manifest as symptoms such as nausea, vomiting, weakness, and in severe cases, renal complications like hypercalciuria and renal stone disease. Hypercalcaemia may occur especially if vitamin D is given in excessive doses or in people with increased sensitivity to vitamin D therapy, such as those with chronic kidney disease, sarcoidosis, tuberculosis, or hyperparathyroidism. If hypercalcaemia is detected, vitamin D (and calcium, if taken) supplements should be stopped and further investigation arranged.

Contraindications for vitamin D replacement therapy include:

• Active renal stones or a history of renal stones, due to the risk of vitamin D toxicity causing hypercalciuria and renal stone disease.
• Severe liver disease or end-stage chronic kidney disease, where specialist treatment with activated vitamin D metabolites may be required instead of standard vitamin D therapy.
• Pregnancy, particularly in the first trimester, where safety data for high-dose vitamin D treatment are lacking and the optimal dose is unclear.

People with medical conditions predisposing to hypercalcaemia may require lower doses and more frequent monitoring, as they are at increased risk of vitamin D toxicity. Additionally, care should be taken in people with malabsorption disorders, who may need individualized replacement schedules under specialist supervision.

Routine monitoring of serum calcium is recommended within 1 month after completing loading doses or starting maintenance therapy to detect hypercalcaemia or hypocalcaemia. If hypocalcaemia is detected, dietary calcium intake should be assessed and increased if inadequate.

Oral vitamin D3 is the preparation of choice for replacement therapy, with vitamin D2 as an alternative in some clinical situations. Intramuscular administration is not recommended first line due to unpredictable bioavailability.

Maintenance doses typically range from 800 to 2000 IU daily, with a maximum of 4000 IU daily in certain conditions such as malabsorption, following specialist advice.