Doppelherz active vitamin D 1000 IU from Queisser Pharma GmbH The immune system faces particular challenges during the damp and cold months, as well as during periods of increased stress, such as from smoking or physical exertion. At these times, there may be a higher demand for certain nutrients. Important information Vitamin D supports the immune system. Our bodies are unable to produce these nutrients themselves. It is important to consume these nutrients regularly through diet. Advice For questions about vitamin D supplementation, a doctor, pharmacist or other healthcare professional can provide advice.