Are food supplements helpful during treatment?

On the whole, as long as you have a balanced diet, you should not need any supplements. They are cleverly marketed, but usually best avoided, particularly if you are taking any prescription medicines as things like magnesium might interact with doxycycline or other antibiotics. Vitamin D in the winter, or throughout the year for those with a dark skin, is probably the only supplement that adults should consider unless they have a definite deficiency. See the British Dietetic Association.