Although vitamins and minerals are essential for good health, the body only needs small amounts. Quite often there is a disproportionate focus on the need for enough or extra vitamins and minerals, which probably has something to do with aggressive supplement company promotions.
Vitamins and minerals are found naturally in food and perform a range of functions which help the body use the energy (calories) found in foods as well as facilitate many metabolic type functions.
In general, a person who eats a balanced and varied diet with enough calories and protein gets plenty of vitamins and minerals. There are of course times when this is difficult in the treatment phase, and in such instances it would be appropriate to discuss with your doctor, the clinical team or the dietitian the use of a low dose multivitamin and mineral supplement.
What I do want to caution you against is the practice of mega-dosing. Often, people with cancer are advised to take, or believe they need large amounts of vitamins, minerals, and other dietary supplements to try to boost everything from their energy levels to their immune system. They may even believe that these supplements can destroy cancer cells. This is not the case; in fact, taking larger doses of supplements can be harmful and there are many studies which associate this practice with a higher incidence of cancer. Large doses of some vitamins and minerals are also known to potentially impact on the effectiveness of chemotherapy and radiation therapy.
The most prudent advice is to aim to eat enough of a variety of food to provide for your requirements. If you are concerned, and if your oncologist agrees, talk to your pharmacist about selecting a supplement with no more than 100% of the Daily Value (DV) of vitamins and minerals.