Why are diuretics dangerous before a competition?

Short answer

Because they shake up potassium along with body water. Bodybuilders who used diuretics right before a show ended up in the emergency room with paralysis and heart rhythm problems from potassium that was too low or too high, and muscle measures and exercise capacity dropped along with the water.

Why are diuretics dangerous before a competition?
'Drain the water → sharper muscles' — at what cost?
Some bodybuilders use diuretics right before a show to drain water under the skin so their muscles look sharper. We checked what these drugs do in the body and what actually happened, using recorded cases and studies.
A drug that flushes out water and sodium
Banned by the World Anti-Doping Agency in and out of competition
Diuretics raise urine output and sodium loss to cut body water. Athletes have used them to drop weight fast or to hide other banned drugs, so the World Anti-Doping Agency (WADA) bans them both during competition and the rest of the year.
The emergency room the day after a show
5-6 kg gone in two days, and he couldn't stand
A 26-year-old pro bodybuilder took a diuretic 24 and 48 hours before a show to look sharper. He lost 5-6 kg through urine at night, and the day after the show he arrived at the emergency room with a pounding heart, unable to move his limbs or stand. His potassium had crashed and his ECG had changed.
Potassium too low or too high was dangerous
Paralysis and heart rhythm problems came together
A 22-year-old bodybuilder who took a diuretic while cutting water saw potassium fall to 1.8, about half the normal range (3.4-4.5); his limbs became paralyzed and his ECG was badly abnormal. A 27-year-old pro who added potassium on top of diuretics had the opposite: potassium rose to a life-threatening level. Another pro who used similar methods died.
Muscle measures dropped along with water
Weight lost fast also cut exercise capacity
10 competitors following one coach's identical diuretic and steroid plan lost body water in show week, and their lean mass (muscle and everything except fat) readings fell too. When 62 people quickly lost 4.1% of body weight with diuretics or a sauna, peak oxygen use and the workload they reached fell compared with losing it through exercise.
Death risk for pro bodybuilders
23 times that of amateurs
Tracking 20,286 men who competed in international shows from 2005 to 2020, 121 died and 46 of those were sudden cardiac deaths. 41 of 1,454 pros (2.8%) died, compared with 80 of 18,832 amateurs (0.4%). The researchers named draining water with diuretics as a heavy strain on the kidneys.
Bottom line: diuretics shake up potassium, not just water
Within days it led to paralysis and heart rhythm problems
Diuretics drain a few kilos of water in a day or two, but cases keep being reported of potassium going too low or too high, sending people to the emergency room with paralysis and heart rhythm problems. Muscle measures and exercise capacity fell too. A 2021 review found almost no experiments showing that show-week water and sodium tricks work.