What are the side effects of sour milk?

November 21, 2025 •

4 min read

According to the U.S. Food and Drug Administration, milk should not be left at room temperature for more than two hours, as harmful bacteria can multiply rapidly. When this occurs, the side effects of sour milk—specifically spoiled milk—can range from mild digestive upset to more serious food poisoning.

Quick Summary

Consuming spoiled, unintentionally sour milk can trigger adverse digestive reactions, such as nausea, vomiting, and cramps, due to harmful bacterial overgrowth. The risks vary depending on the amount ingested and the type of bacteria present. Proper identification of spoiled milk is crucial for preventing illness.

Key Points

Spoiled vs. Soured Milk: The terms are not interchangeable; spoiled milk is unsafe to drink due to harmful bacterial growth, while intentionally soured milk (like buttermilk) is safe.
Acute Digestive Symptoms: Drinking spoiled milk often leads to acute digestive distress, including nausea, vomiting, abdominal cramps, and diarrhea.
Risk of Food Poisoning: Harmful bacteria such as E. coli and Salmonella can proliferate in spoiled milk, causing foodborne illnesses.
Not a Long-Term Risk (Generally): For healthy individuals, symptoms from small quantities of spoiled milk are usually mild and resolve within 12-24 hours.
High-Risk Populations: Infants, the elderly, and immunocompromised individuals are at a much higher risk for severe complications from spoiled milk.
Trust Your Senses: Rely on smell and appearance (off-odor, curdling, yellowing) rather than just the expiration date to determine if milk is safe.
Raw Milk is Dangerous: Raw, unpasteurized milk should never be consumed when sour due to a high risk of dangerous pathogens.

Distinguishing Intentionally Soured vs. Spoiled Milk

It is critical to understand the difference between intentionally soured dairy products and spoiled milk. Intentionally soured products, like buttermilk and kefir, are created through a controlled process using specific, safe bacteria. Spoiled milk, on the other hand, is the result of uncontrolled bacterial contamination and is not safe for consumption.

Why Milk Spoils

Milk is a highly nutritious medium, which makes it an ideal breeding ground for both beneficial and harmful microorganisms. Most milk sold in the United States is pasteurized, a process that heats the milk to kill most of these harmful pathogens. However, pasteurization does not eliminate all bacteria. Over time, or if milk is left unrefrigerated, the remaining bacteria multiply, producing lactic acid that causes the characteristic sour taste, smell, and curdled texture.

The Impact of Consuming Spoiled Milk

Drinking spoiled milk can introduce a variety of harmful bacteria into your digestive system, such as E. coli, Salmonella, and Listeria. The severity of the side effects depends on factors like the amount consumed, the specific bacteria involved, and the individual's overall health.

Acute Side Effects of Spoiled Milk

For most healthy adults, ingesting a small amount of spoiled milk may result in a bad taste with no further symptoms. However, larger quantities can cause acute digestive distress as the body works to eliminate the contaminants.

Gastrointestinal Distress: The most common reactions include nausea, abdominal pain, stomach cramps, and vomiting. The body's natural response is to expel the irritating bacteria and toxins.
Diarrhea: This is another common symptom, which can lead to dehydration. It's crucial to replenish fluids and electrolytes if experiencing significant diarrhea or vomiting.
Fever and Chills: In more severe cases of foodborne illness, the body's immune response can lead to chills and a fever.

Chronic and Long-Term Health Risks

While most cases of spoiled milk ingestion are resolved within 12-24 hours, repeated exposure or particularly virulent bacteria can pose chronic risks, including a weakened immune system or ongoing stomach issues. Vulnerable populations, such as the very young, elderly, and immunocompromised, face higher risks of serious illness or hospitalization.

How to Tell If Milk is Spoiled

Before consuming milk, always check for signs of spoilage. While expiration dates are a useful guide, they are not a guarantee of safety. The best indicators are your senses.

Smell: A strong, sour, or rancid odor is a clear sign that milk is spoiled.
Appearance: Fresh milk is smooth and white. Spoiled milk may appear thick, curdled, lumpy, or have a yellowish tint.
Taste: If the milk smells and looks fine but you're still unsure, a small taste can confirm if it's off. A sour or acidic taste is a definite warning sign.

Comparison Table: Intentionally Soured vs. Spoiled Milk


Feature	Intentionally Soured Milk (e.g., Buttermilk)	Spoiled Milk
Safety	Perfectly safe for consumption.	Unsafe for consumption; can cause food poisoning.
Origin	Produced under controlled conditions with beneficial bacteria or acid.	Result of uncontrolled growth of harmful bacteria.
Taste	Tangy and acidic due to lactic acid.	Off-tasting, excessively sour, or rancid.
Texture	Thicker and sometimes slightly curdled intentionally.	Lumpy, slimy, or significantly curdled.
Usage	Used for baking, marinades, or in recipes.	Should be discarded to prevent illness.

What to Do If You Accidentally Drink Spoiled Milk

If you accidentally ingest a sip or a small amount of spoiled milk, don't panic. For most people, it's unlikely to cause severe harm beyond an unpleasant taste. Rinsing your mouth and drinking some water can help. If you have consumed a significant quantity, it's important to monitor for symptoms like vomiting, cramps, or diarrhea. Hydrate with fluids containing sugar and electrolytes, like sports drinks or broth. If symptoms worsen, become severe, or last for more than 24 hours, contact a healthcare provider.

The Special Case of Raw Milk

Raw, unpasteurized milk carries a much higher risk of containing dangerous pathogens. Even when it is intentionally soured through natural fermentation, it is not recommended for consumption due to the high risk of foodborne illness. The fermentation of raw milk can create bacteria that do not offer the same probiotic benefits as controlled cultures and pose a serious health risk, particularly to vulnerable individuals. For this reason, federal health experts advise against consuming raw milk or raw milk products. For a complete list of recommended food safety practices, consult the official guidelines provided by FoodSafety.gov.

Conclusion

While a slightly sour but still-edible batch of milk can be repurposed for baking, milk that has truly spoiled from harmful bacterial growth should always be discarded. The side effects of sour milk, when spoiled, are primarily gastrointestinal and can be severe for certain groups. Relying on your senses of smell and sight is the most effective way to prevent the risks associated with consuming bad milk. When in doubt, it's always safest to throw it out and opt for a fresh carton.

Frequently Asked Questions

If you drink a small sip of spoiled milk, you will likely experience nothing more than an unpleasant taste. A rinse of the mouth and a drink of water is usually sufficient. Severe symptoms are more likely with larger quantities.

Intentionally soured milk (like buttermilk) is made in a controlled process and is safe to consume, while spoiled milk results from uncontrolled bacterial growth and is unsafe. Spoiled milk will often have a rancid smell, lumpy texture, and off-taste.

The most common digestive issues include abdominal pain, nausea, vomiting, and diarrhea. These are caused by the body's reaction to harmful bacteria and toxins in the spoiled milk.

If milk is only slightly off and not showing major signs of spoilage like mold or curdling, it can sometimes be used for baking where heat will kill most bacteria. However, it may impart an off-flavor to your food, so when in doubt, it's safer to discard it.

Symptoms of food poisoning from spoiled milk can appear within a few hours to 12-24 hours after consumption.

While unlikely for a single instance in healthy individuals, repeat exposure to harmful bacteria or toxins in spoiled milk could potentially lead to chronic issues over time, such as ongoing stomach problems or weakened immune function.

You should see a doctor if symptoms like vomiting or diarrhea become severe, you experience signs of dehydration, have a high fever, or symptoms last for more than 24 hours.