If you learn one thing about milk, let it be this: Unlike cheese, it doesn’t (I repeat, does not) get better with age. Drinking a tall pour of spoiled milk from a weeks-old gallon in the back of your fridge is not a great idea. Since food poisoning is rarely the goal when consuming dairy, I figured it was high time to uncover the why behind spoiled milk—and the gut reaction that follows. Keep reading to learn what happens if you drink spoiled milk. 

Why does milk spoil?

“Milk curdles in part because of bacteria,” says gastroenterologist and internist Niket Sonpal, MD. “Even if milk is pasteurized, there’s still a certain amount of milk bacteria left behind that will eventually cause the milk to spoil and curdle.” 

According to 2018 research published by the Journal of Dairy Science, this is because the bacteria continues to grow even after the milk has been pasteurized¹ (which involves heating up the milk to kill off pathogens) and bottled. Yuck. Milk will go bad regardless, but you can slow the process by storing your carton at 38°F to 40°F.

How do you know milk has gone bad?

Although refrigerating milk slows the process, it doesn’t fully prevent it from spoiling. According to NYC-based dietitian Jennifer Maeng, MS, RD, opened milk that’s kept in a refrigerator typically goes bad within four to seven days of the printed best-by date. “If milk is unopened and left in the refrigerator, it can usually last an extra five to 10 days past the printed date, depending on the fat percentage of the milk,” she adds. 

Mind you, this calls into question the difference between sell-by, best-by, and expiration dates—which the FDA doesn’t regulate.  Depending on the state you live in, the label may have different requirements. While some states insist on expiration dates (which detail the exact date when a product is deemed bad), others are more lenient with sell-by and best-by dates. If your state falls into the latter category, you can apply Maeng’s advice. If, however, your state uses expiration dates, it’s best not to consume milk past that printed date. 

No matter the label, if one sniff of your milk should lead you to utter “hmmm, this milk smells funky”—that’s a pretty clear indicator that your milk has gone bad. The best case scenario is that you’ll throw it out immediately after noticing the smell. If, however, you pour the spoiled milk into your Lucky Charms, smoothie, or protein shake, and gulp it down… there’s no gentle way to say this: you will likely get food poisoning.

What are the signs of spoiled milk?

Although a funky smell is one of the first noticeable signs of spoiled milk, it’s not the only indicator. So, what happens when milk spoils? According to Maeng, a sour taste, change in color, and/or change in color are also telltale signs that your milk has gone bad. (And trust me, there’s nothing more unsettling than pouring what you assume to be delicious, ice-cold milk, only to find chunks flop out in your bowl—it’s something that will stay with you so steadfastly that you’ll never overlook your milk’s date or appearance again.)

What is the difference between sour milk and spoiled milk?

Newsflash: sour and spoiled are not synonymous when it comes to milk. “Sour milk is a dairy product that is safe to consume, while spoiled milk is not safe to consume,” Maeng says. “Sour milk is produced by either a fermentation process or by adding an acid, such as vinegar or lemon juice, into the milk. Both processes cause the milk to increase in acidity, giving it a sour taste, and to congeal. When the milk congeals, it hinders the growth of any unfavorable bacteria in it, thus making it safe to consume.”

Meanwhile, spoiled milk is milk that has gone bad, either because it’s well past its printed date, or because it was accidentally left unrefrigerated. It can go bad within just two hours outside of the fridge, Maeng says, noting that it’s a common food storage refrigerator mistake to avoid.

While on the topic, sour and spoiled milk are just two stages of dairy. According to Maeng, there’s also raw and curdled milk. “Raw milk is milk that has not undergone pasteurization,” she explains. “Therefore, it contains a lot of pathogenic bacteria and poses a high risk to those who consume it.”

Then there’s curdled milk, which can be tricky. “Curdled milk is a sign of spoilage, but that does not mean all curdled milk is unsafe to consume,” Maeng says. “Milk curdles for several reasons, one being when combined with lemon juice or vinegar for a recipe or when being added to highly acidic coffee or tea. In these cases, curdled milk is safe to consume. Just make sure the milk was not curdled, to begin with.”

What are the risks of drinking spoiled milk?

“Typically, if you ingest enough spoiled milk, it can take a toll on your gastrointestinal tract,” Dr. Sonpal says. So, depending on how much expired milk you sipped, your symptoms may include vomiting, cramping, and diarrhea for a period of time from hours to days, says the gastroenterologist.

How long after drinking bad milk do you get sick?

If you only have a sip or two, Maeng says that it’s perfectly possible not to experience any adverse reaction. (Other than wanting to immediately spit it out.) But even if you actually ingest the spoiled milk—like a small cup’s worth or what you mix with your cereal—it’s unlikely to make you sick. 

What do I do if I drank spoiled milk?

If you don’t experience any symptoms, nothing. You’re fine.

Mild cases of food poisoning usually pass on their own in due time. Dr. Sonpal recommends making sure you’re staying hydrated so your kidneys don’t suffer. However, if you’re trying to stay hydrated, but you keep vomiting up the Pedialyte or Gatorade that you’re trying to drink, you might be experiencing more severe symptoms, says Dr. Sonpal. If this is the case, you should go to urgent care or your doctor’s office so they can keep you hydrated with IV fluid.

What bacteria is in spoiled milk?

The main reason why spoiled milk can be gut-wrenching (literally) is due to the bacteria found within it. Dr. Sonpal notes that both salmonella and E. coli have been found in pasteurized milks. Meaning if you do get sick after gulping down a glass, it can’t hurt to go to your doctor’s office just in case. And as a hard-and-fast rule, he recommends avoiding milk that hasn’t been pasteurized—no matter your age.

Dairy-free milk can spoil, too

Although cow’s milk tends to be the culprit of spoilage, all of milk alternatives (like soy milk, oat milk, and almond milk) can go bad, too. You should always pay attention to your milk’s label, smell, and appearance—whether it’s dairy or an alternative—before consuming it. (That being said, alt-milks tend to last a long longer than cow’s milk…especially when left unopened. Ultra-filtered milk is also more shelf-stable than regular unopened cow’s milk, thanks to the filtration process and pasteurization it goes through. Still, you’ll want to consume it within seven to 14 days.) 

Is it safe to drink expired powdered milk?

If you’re relying on powdered milk in your household, know that these products have a much longer shelf life—typically 12 to 24 months. Even once those dates pass, the common conclusion is that it’s safe to drink powdered milk so long as it hasn’t changed color or developed an odd smell. 

The final word

You don’t want to drink spoiled milk if you can help it. The easiest way to avoid doing so is to check your milk labels regularly. Some people are weary of using milk past its “best by” or “sell by” date, simply because they never want take the chance of sipping on spoiled dairy. That said, Maeng says it’s not necessary to toss your milk as soon as the best-by or sell-by date passes. “Instead, wait for signs of spoilage,” she says.

Yes, we all learned as children not to cry over spilled milk. Spoiled milk, however, is a different story. Do yourself a favor and take a whiff before you drink a glass.

An RD shares an in-depth guide to alternative milks: