There was a news article this week about a man who became very ill after taking green tea pills. This previously healthy individual now needs a liver transplant after taking the supplement.
Whilst this is an isolated case, it should be pointed out that there are more cases each year of adverse events to medications (250,000 hospital admissions) than there are to vitamin supplements.
A supplement is exactly that, and should supplement the food you are eating. They are not designed to take the place of a healthy diet. It is important to remember that the reason this news story hit the headlines is that it was a very rare occurrence, and supplements are generally a low risk products with possible high benefits.
Whilst it is always best to get our nutrition from food, sometimes there are situations where taking a supplement is advised, for example, a strict vegan might be recommended to take a vitamin B12 supplement as this nutrient is mainly found in animal-based foods.
If you feel you need to take a supplement, make sure the brand you are using is reputable and that you take the recommended dose. Check on the packaging for any health disclaimers, for example, some supplements are not suitable for use in pregnancy.
A registered nutritional therapist would be able to check if there are any drug-nutrient interactions with any medication you are taking.