Eating spoiled olives typically causes a bout of food poisoning with nausea, vomiting, stomach cramps, and diarrhea that resolves within a day or two. In rare cases, particularly with homemade or improperly stored olives, the consequences can be more serious, including botulism. What actually happens to you depends on what went wrong with the olives and how much you ate.
Mild Cases: Standard Food Poisoning
Most of the time, eating a few bad olives leads to the same kind of illness you’d get from any spoiled food. Your stomach and intestines react to the bacteria or their toxic byproducts, and you experience some combination of nausea, vomiting, diarrhea, and abdominal cramps. This usually starts within a few hours of eating and clears up on its own within one to three days.
Spoiled olives can harbor several common foodborne bacteria, including E. coli, Listeria, and Staphylococcus aureus. These are the same organisms responsible for most everyday food poisoning cases. The illness they cause is unpleasant but rarely dangerous for otherwise healthy adults.
Biogenic Amines: Headaches and Beyond
Fermented olives that have gone bad can accumulate compounds called biogenic amines, which form as proteins break down during spoilage. These substances cause a distinct set of symptoms that go beyond typical food poisoning: migraines, headaches, diarrhea, and even insomnia or low mood. One telltale sign of this type of spoilage in green olives is called “zapatera,” which produces a noticeably foul, cheesy smell.
If you’ve ever gotten a headache after eating olives that tasted slightly off, biogenic amines are a likely explanation. The reaction is more of a chemical intoxication than an infection, and it generally passes once your body clears the compounds.
Botulism: The Rare but Serious Risk
The most dangerous thing that can grow in spoiled olives is Clostridium botulinum, the bacterium that causes botulism. This is considered the single most relevant safety hazard in table olives. Botulism is rare, but it’s a medical emergency that can cause muscle weakness, difficulty swallowing, blurred vision, and in severe cases, paralysis of the muscles you use to breathe.
Botulism becomes a risk when olives are stored in low-oxygen environments (like sealed jars or submerged in oil) with a pH above 4.6 and at room temperature. Commercially produced olives are carefully acidified to prevent this. The real danger comes from artisanal or homemade preparations where the acidity isn’t properly controlled. Even tiny water droplets trapped in oil can provide enough moisture for the toxin to form. One particularly unsettling detail: olives contaminated with botulinum toxin don’t look, smell, or taste any different from safe ones.
Mold and Mycotoxins
If your olives have visible fuzzy mold on them, the risk goes beyond just an unpleasant texture. Certain molds that grow on olives, particularly species of Aspergillus and Penicillium, produce mycotoxins. These toxic compounds can cause skin irritation, suppress immune function, and in cases of prolonged exposure, contribute to more serious health problems. Cracked olives and Greek-style black olives are especially susceptible to mycotoxin contamination.
You may also notice a white film floating on the brine. This is often “kahm yeast,” a mixture of oxidative yeasts and bacteria that forms on the surface when olives are exposed to air. While kahm yeast itself isn’t immediately dangerous, it breaks down the acids that keep olives safe. As the brine becomes less acidic, the pH rises, and conditions become hospitable for harmful bacteria and mycotoxin-producing molds. So a white film is a warning sign that the olives’ safety barrier is eroding, even if the film itself looks harmless.
Rancid Olives and Oxidized Fats
Sometimes olives aren’t contaminated with bacteria or mold but have simply gone rancid. The fats in olives oxidize over time, especially when exposed to heat, light, or air. Rancid olives taste sharp, waxy, or like old crayons. Eating a small amount won’t send you to the hospital, but regularly consuming oxidized fats is worth avoiding. Oxidized lipids can damage the cells lining your intestines, trigger inflammation, and disrupt normal cellular function in your gut. Over time, this kind of chronic low-level damage is linked to inflammatory intestinal conditions.
How To Spot Bad Olives
Before you eat olives that have been sitting around, check for these signs:
- Smell: If the brine or oil smells putrid, sour in an unusual way, or just “off,” discard them.
- Texture: Olives that have become mushy or slimy have broken down past the point of safety.
- Visible mold: Any fuzzy growth on the olives means they should be thrown away immediately.
- Packaging damage: Dented cans, bulging lids, or torn vacuum seals all indicate the olives may be compromised. A bulging lid is an especially concerning sign because it suggests gas-producing bacteria are active inside.
- Taste: If the olives pass the visual and smell checks but taste noticeably wrong, spit them out and toss the rest.
How Long Olives Stay Safe
Unopened jarred olives stay fresh for up to two years and don’t need refrigeration. Once opened, they last 12 to 18 months if you keep them sealed and refrigerated with their brine or oil covering them. Olives from a deli olive bar have a much shorter window: about three weeks in the fridge.
Temperature matters more than most people realize. Keeping olives below 38°F (3.3°C) inhibits the growth of even cold-tolerant strains of botulinum bacteria. A standard refrigerator set to its recommended range handles this, but olives left out on a counter or stored in a warm pantry after opening lose their safety margin quickly.
Symptoms That Need Medical Attention
Most food poisoning from bad olives will pass on its own. But certain symptoms signal something more serious is happening. Bloody diarrhea, a fever above 102°F, vomiting so frequent you can’t keep liquids down, or diarrhea lasting more than three days all warrant a call to your doctor. Signs of dehydration, like dizziness when standing, a dry mouth, or not urinating much, also need attention.
If you develop any neurological symptoms after eating olives, especially difficulty swallowing, blurred or double vision, drooping eyelids, or muscle weakness, seek emergency care. These are hallmarks of botulism, which requires immediate treatment. Pregnant women who develop a fever and flu-like symptoms after eating questionable olives should also contact their doctor promptly, as Listeria infection carries specific risks during pregnancy.