Short answer: yes. Cooking oil doesn't grow mould or "spoil" the way milk does, but it absolutely goes off — the process is called rancidity, and once you learn to recognise it you'll find it hiding in more kitchen cupboards than you'd think. Here's how to tell, how long different oils really last, and the storage habits that can double an oil's useful life.

What actually happens when oil goes off

Oils are fats, and fats oxidise. Light, heat and oxygen slowly break their fatty acids apart, producing the stale, sharp compounds you can smell and taste. Nothing dramatic happens on a particular date — it's a slide, not a cliff — which is why an oil can be technically "in date" and still taste tired, or a well-kept bottle can outlive its label.

The speed of that slide depends mostly on the oil's fat profile: delicate polyunsaturated fats (the omega-3s in flaxseed or walnut oil) oxidise fastest, while stable monounsaturated oils (high-oleic sunflower, avocado) hold out much longer.

The crayon test: how to tell if oil is rancid

• Smell it. Rancid oil smells like crayons, window putty or old paint. Fresh oil smells like its source — nuts, seeds, fruit — or of very little at all.
• Taste a drop. Harsh, bitter, scratchy at the back of the throat — that's oxidation, not "character".
• Look, last. Colour changes and stickiness around the cap are late signs; trust your nose first.

Is rancid oil dangerous? Eating it once won't send anyone to hospital — the bigger issue is that it tastes unpleasant and its beneficial compounds have already degraded. Either way: once it smells of crayons, it's done.

How long do oils actually last?

Always trust the best-before date on the bottle — a serious producer counts it from the pressing date. As a rule of thumb:

Notice the pattern: opening the bottle starts a much faster clock than the label date. That's the single most misunderstood thing about oil freshness.

Five storage habits that genuinely help

• Dark beats light. Light is oil's worst enemy — dark glass helps, a closed cupboard helps more. Never store oil on a sunny windowsill.
• Away from the hob. The cupboard right next to the cooker bathes your oil in heat every day. Two metres makes a real difference.
• Cap on, tight, immediately. Every hour open to the air is oxidation time.
• Fridge for the delicate ones. Flaxseed, hemp, walnut and other omega-3-rich oils live longest chilled. Cloudiness or thickening in the fridge is completely normal — it clears at room temperature.
• Buy the size you'll finish. A 250ml bottle used in two months beats a litre used in a year, every time. It's the whole logic behind small-format cold pressed oils.

Does unopened oil expire?

An unopened bottle past its best-before isn't automatically rancid — it's just past the window the producer will vouch for. Well stored, a stable oil may be fine months later; a delicate one probably isn't. Apply the crayon test and let your nose decide.

Frequently asked questions

Can you use oil after the best-before date?
If it smells and tastes clean, it's usable — quality, not safety, is what the date protects. Delicate oils like flaxseed deserve more caution than stable ones.

Why has my oil gone cloudy in the fridge?
Natural waxes and saturated fats solidify in the cold. It's harmless and reverses at room temperature.

Can you freeze cooking oil?
You can, but for most oils the fridge or a dark cupboard achieves the same result with less faff.

Does rancid oil make you ill?
A one-off exposure is unlikely to cause harm, but rancid oil tastes bad and its nutritional value has degraded — replace it.

The short version

Oil goes off gradually — by light, heat and air — and your nose is the best detector ever made. Keep bottles dark, cool and tightly capped, refrigerate the delicate ones, and buy sizes you'll finish while they're alive. For which oils tolerate heat in the first place, see our smoke point guide; for why unrefined oils are worth this small care, start with what cold pressed actually means.