Why has my e-liquid changed colour? Is it still OK?

Pure nicotine is close to colourless, but it reacts with oxygen and light. As it oxidises it moves from clear to pale yellow, then towards amber and brown. The more nicotine a liquid has, the more noticeable the change tends to be, which is why a 20mg salt can darken faster than a 3mg shortfill.

It happens slowly in a sealed bottle and faster once opened, because every refill lets in fresh air. A bottle that sat on a sunny desk will darken far quicker than one that lives in a drawer.

Nicotine-free liquid can still change colour, though, so nicotine is not the only factor.

Some flavourings are famous for browning. Vanillin and ethyl vanillin, the backbone of most vanilla, custard and cream liquids, darken noticeably over weeks. Caramel, toffee, coffee and tobacco notes are often amber from the start and get deeper with time.

Clean fruit and menthol liquids usually stay paler for longer, although a high nicotine version will still yellow. So if your custard has turned the colour of weak tea while your mint is still clear, that is completely normal chemistry rather than a fault. The vanilla flavours range is a good place to see this in action.

Three things push colour change along: light, heat and air. Sunlight is the worst, especially through a window. Heat from radiators, cars or kitchen appliances speeds up every reaction. And air, let in each time you open the bottle, feeds the oxidation.

Even the liquid in your pod can darken over a few days, because it sits close to a warm coil and is exposed to air through the airflow. That is one reason a tank of liquid that has been sitting for a week can taste slightly different from a fresh fill. Storing bottles properly, as covered in how to store e-liquid, slows all of this down.

Colour on its own tells you very little. Plenty of perfectly good liquids are amber, and a gentle darkening over a few weeks is expected. What matters more is the date on the label, the smell and the taste.

If the liquid is within its best before date, smells like it should and tastes fine, carry on. If it smells sour or chemical, has gone cloudy or has bits in it, throw it away. If it is past its date, replace it: nicotine can degrade, so the strength may no longer match the label. Answer below for a quick verdict on your bottle.

Darker liquid sets off alarm bells for a lot of people, and you will hear some fairly wild theories about why it happens. Most of them confuse normal ageing with spoilage, or assume colour tells you the strength.

The truth is less dramatic. Colour is a clue about age and exposure to light and air, not a reliable test of whether a liquid is good or bad. Flip each card to see what is really going on, then use your nose and the label rather than your eyes to decide. Shopping for paler, cleaner flavours? Try spearmint.

Here is a real situation we get asked about all the time. You bought a vanilla custard shortfill three months ago, added a nic shot, and kept it capped in a kitchen drawer. It started pale cream and is now a deep amber, almost like cold tea.

The best before date on the label is next year, it smells exactly as it always did and it tastes fine, maybe even a little richer. What is going on with the bottle? Think about which ingredient in a vanilla custard is known for browning, and whether colour on its own is ever enough to judge a liquid.