What could be more refreshing and harmless than a juicy watermelon on a hot summer day?

This symbol of health and freshness can actually pose a real threat to people with certain medical conditions. And we’re not just talking about the risk of nitrate poisoning.

While most of us enjoy its sweet flesh without a second thought, for some people this popular summer treat is a serious red flag. Doctors warn: this seemingly innocent fruit can place a heavy burden on the body. If you suffer from certain chronic illnesses, it’s best to avoid watermelon altogether—or at least eat it in very small amounts.

The Main Risk: Your Kidneys

Medical and nutrition experts agree: the number one risk group is people with chronic kidney disease.

“Patients with chronic kidney conditions should be very cautious when consuming watermelon,” doctors emphasize.

Why is it so dangerous? There are two main reasons:

Fluid overload. Watermelon is about 90% water. Its famous diuretic effect puts tremendous strain on the urinary system. Healthy kidneys can handle this without issue. But when kidney function is impaired (for example, in kidney failure), they can’t remove such a large amount of fluid. This may lead to swelling and increased blood pressure.

Excess potassium. Watermelon is rich in potassium. This mineral is essential for heart and muscle health, but too much of it (a condition known as hyperkalemia) can be dangerous. Healthy kidneys easily flush out extra potassium. Diseased kidneys cannot, which causes potassium levels in the blood to rise. This can lead to muscle weakness, and in severe cases, dangerous heart rhythm disturbances—even cardiac arrest.

Who Else Should Be Careful?

Kidney problems aren’t the only concern. Other groups of people should also treat watermelon with caution:

People with diabetes. Watermelon has a high glycemic index. The sugar it contains is absorbed very quickly, causing a sharp spike in blood glucose. For people with type 1 or type 2 diabetes, this can be dangerous. If eaten at all, it should be in strictly limited amounts, with careful monitoring of carb intake.

People with digestive issues. For those with irritable bowel syndrome (IBS), colitis, or during flare-ups of pancreatitis, watermelon can trigger discomfort. Its high fiber and fructose content may cause bloating, gas, and diarrhea.

People with large kidney stones. Watermelon’s diuretic effect can cause stones to move, potentially triggering a painful attack of renal colic.

Watermelon: The Benefits and the Risks

To make it clear, let’s sum it up.

Benefits (for healthy people):

Excellent hydration — the best way to replenish fluids in the heat.

Source of lycopene — a powerful antioxidant that gives watermelon its red color, helps protect the heart, and reduces cancer risk.

Rich in vitamins — contains vitamin C for immunity and vitamin A for vision and skin.

Good for blood vessels — provides citrulline, an amino acid that improves circulation and may help lower blood pressure.

Low in calories — a perfect dessert for those watching their weight.

Risks and drawbacks:

Dangerous for kidney disease — overloads the body with water and potassium.

Raises blood sugar — its high glycemic index makes it risky for diabetics.

Can upset digestion — may cause bloating and diarrhea in people with sensitive GI tracts.

Risk of nitrate poisoning — especially with “early” watermelons bought out of season (safe season is late August to September).

The Bottom Line

Watermelon is a wonderful, healthy fruit—but only for healthy people, and only in moderation. If you have chronic illnesses, especially kidney disease, talk to your doctor before indulging in this summertime favorite.