The best time to take a diuretic is when you have the most symptoms. If your symptoms are mild, you can take your medication at any time of day. However, if your symptoms worsen or do not respond to the medication quickly then you should contact your doctor or pharmacist immediately. Diuretics do work more effectively on an empty stomach but some people still find that they need to eat something before taking their diuretic tablets for best results. So if you are feeling anxious about taking diuretics because of this factor, try eating one small meal first and see how it works out for you.