The most common serious side effect of spironolactone is a high level of potassium in the blood. This is called hyperkalemia. If it is very high, it can cause heart rhythm problems. Dangerous levels can happen in 1% to 12% of people taking spironolactone. It is more common if you are taking other drugs that raise your potassium or if you have kidney problems. Your doctor will test your potassium levels regularly when you are on spironolactone.
Other serious side effects are very rare. Spironolactone can cause bleeding in the digestive tract. It can also cause severe skin reactions. Some people have a drop in the number of infection-fighting white blood cells, but this is very rare, too.