Diuretics are generally safe. Side effects include increased urination and sodium loss. Diuretics can also affect blood potassium levels. If you take a thiazide diuretic, your potassium level can drop too low (hypokalemia), which can cause life-threatening problems with your heartbeat.
https://www.mayoclinic.org/diseases-conditions/high-blood-pressure/in-depth/diuretics/art-20048129