The surest way to know if you need to take a vitamin D supplement is to use a Vitamin D Test Kit. This is a home blood test that can read your vitamin D levels to indicate if you’re likely to benefit from taking a vitamin D supplement. Regardless, Public Health England (PHE) recommends that everyone considers taking a daily 400IU (10mg) vitamin D supplement in autumn and winter, as there tends to be less sunlight during these months. According to PHE, supplements providing up to 25 micrograms (1000 IU) are suitable for everyone.