Potassium-depleting diuretics, including loop diuretics, cause the body to lose potassium. Loop diuretics may also cause cellular magnesium depletion,¹ although this deficiency may not be reflected by a low blood level of magnesium.² Magnesium loss induced by potassium-depleting diuretics can cause additional potassium loss. Until more is known, it has been suggested that people taking potassium-depleting diuretics, including loop diuretics, should supplement both potassium and magnesium.³

People taking loop diuretics should be monitored by their doctor, who will prescribe potassium supplements if needed. Such supplementation is particularly critical before surgery in patients with a history of heart disease. In a preliminary study, people with low blood levels of potassium (in part related to diuretic use) had a higher incidence of serious problems resulting from surgery (including death) compared with those having normal potassium levels.⁴ Fruit is high in potassium, and increasing fruit intake is another way of supplementing potassium. Magnesium supplementation is typically 300–400 mg per day.

One study showed that people taking diuretics for more than six months had dramatically lower blood levels of folic acid and higher levels of homocysteine compared with individuals not taking diuretics.⁵ Homocysteine, a toxic amino acid byproduct, has been associated with atherosclerosis . Until further information is available, people taking diuretics for longer than six months should probably supplement with folic acid.

People with congestive heart failure (CHF) treated with the loop diuretic furosemide may be at risk for vitamin B1 deficiency due to: 1) the disease, 2) treatment with furosemide, and/or 3) inadequate dietary vitamin B1 intake.⁶ In a study of people with CHF, long-term furosemide therapy was associated with clinically significant vitamin B1 deficiency due to urinary losses.⁷ This furosemide-induced vitamin B1 deficiency may worsen heart function in patients with CHF and may be prevented or corrected with vitamin B1 supplementation.⁸

Herbs that have a diuretic effect should be avoided when taking diuretic medications, as they may enhance the effect of these drugs and lead to possible cardiovascular side effects. These herbs include dandelion , uva ursi , juniper , buchu , cleavers , horsetail , and gravel root.⁹

Use buckthorn (Rhamnus catartica, Rhamnus frangula, Frangula alnus) or alder buckthorn for more than ten days consecutively may cause a loss of electrolytes (especially the mineral potassium). Medications that also cause potassium loss, such as some diuretics, should be used with caution when taking buckthorn or alder buckthorn.¹⁰

Diuretics, including loop diuretics, cause increased loss of sodium in the urine. By removing sodium from the body, diuretics also cause water to leave the body. This reduction of body water is the purpose of taking diuretics. Therefore, there is usually no reason to replace lost sodium, although strict limitation of salt intake in combination with the actions of diuretics can sometimes cause excessive sodium depletion. On the other hand, people who restrict sodium intake and in the process reduce blood pressure may need to have their dose of diuretics lowered.