Diuretics can result in various undesired biochemical changes, such as impotence, skin rashes, nausea, dizziness and lethargy as well as subjective side effects. The side effects are mostly predictable, their effects depending on both the circulatory blood volume and on the transport of water and solute in the renal tubules. Two of the commonest side effects are mild hypovolaemia, when any diuretic is used, and mild hypokalaemia when the non-potassium-sparing diuretics, such as thiazides and frusemide are used. Its occurrence is dose dependent and can be corrected by potassium supplements, but potassium-retaining diuretics, which also correct the often associated fall in serum magnesium, are preferable. Many reports link hypokalaemia with cardiac arrhythmias, but some dispute this association in the absence of the concomitant use of digoxin. Hyponatraemia rarely occurs, but can be life threatening. Calcium excretion is markedly reduced, but unlike other electrolyte disturbances from diuretics, this may be valuable: some suggest diuretics have an anti-osteoporotic action. Diuretics increase glucose and insulin resistance and should be used sparingly in diabetics. They rarely cause a non-ketotic hyperosmolar coma. Urate is raised, but clinical gout is not common. Cholesterol elevation has been reported in some studies, but long-term studies indicate that lipid changes are minor. Other rare side effects are not predictable from their pharmacological actions and these include the occurrence of skin rashes, thrombocytopenia, pancreatitis and interstitial nephritis; and ototoxicity from frusemide.