Yes — vitamin D and calcium are routinely taken together (vitamin D helps your body absorb and use calcium), but stay within recommended doses and check with your pharmacist, especially about total vitamin D from all your supplements.
These two work as a pair rather than against each other: vitamin D helps your body absorb and regulate calcium, which is why they are commonly combined in supplements for bone health. The risk is not the pairing itself but the dose — taking too many vitamin D supplements over a long period causes calcium to build up in the blood (hypercalcaemia), which can weaken bones and damage the kidneys and heart, and excess calcium can cause stomach upset. Per NHS guidance, most adults need only about 10 micrograms (400 IU) of vitamin D and 700 mg of calcium a day, so a sensible combined supplement is well within safe limits.
Keep within the upper limits: NHS advises adults not exceed 100 micrograms (4,000 IU) of vitamin D a day and that 1,500 mg or less of calcium daily is unlikely to cause harm (700 mg is the target). Check the labels of ALL your supplements and multivitamins for vitamin D, since it is easy to double up without realizing. Symptoms of too much vitamin D / high blood calcium include nausea, vomiting, constipation, appetite loss, excessive thirst and urination, confusion, and muscle weakness — stop and seek medical advice if these appear. Excess calcium can also cause stomach pain and diarrhoea. Talk to your doctor or pharmacist before combining these if you have kidney disease or kidney stones, take thiazide diuretics or certain heart medications, have a condition causing high calcium, or are taking high-dose (prescription) vitamin D — and have blood calcium checked if you are on high doses.
This is general reference, not medical advice, and not a guarantee of safety. Interactions depend on your doses, health conditions, and other medicines. Always confirm with your pharmacist or doctor before combining products, and follow the dosing on each label.