Yes, there is a downside to taking vitamin D3. While it is true that this nutrient can offer a number of health benefits, including helping to improve bone health and reducing the risk of certain cancers, it can also cause some negative side effects. These include an increased risk of kidney stones, as well as gastrointestinal issues such as constipation or diarrhea. Additionally, too much vitamin D3 can lead to toxicity, which can cause serious problems such as weakness, confusion, and even death.