Extreme overdose, i.e. blood concentrations of more than 150 ng / ml vitamin D, may cause dizziness, vomiting, muscle weakness, diarrhea and eventually long-term gastric ulcers. At the commonly recommended dosage of 2 capsules per day, i.e. 2,000 IU (international units), a Vitamin D supplement stabilises your vitamin D level, e.g. as a preventive in the winter months, but will not increase it.
It is advisable to monitor your vitamin D value and in general to have it tested about twice a year and readjust your dosage, since your blood concentration generally decreases over the course of the year even when taking a maintenance dose.