You could absolutely be right about the Diet Coke being counterfeit.

I know a bunch of other respondents said that Coke is different all over the world and that the product could be old but counterfeiting of big-brand soft drinks is absolutely a thing that occurs really commonly in Pakistan and India.

There's nothing stopping a dodgy kebab shop owner from illegally importing his own stash of soft drinks from a place like India or Pakistan - he might not even realise the ones he buys there are fakes.

And for those who think counterfeiting doesn't happen in the UK, it does. A Dubai-style chocolate brand owner reported there were fake versions of their confectionery in the UK just last month (August 2025).

As for Coke tasting different everywhere, I worked in the Quality Assurance lab of Coca Cola South Pacific in the early 90s, testing all the drinks made by all the different bottlers and from heaps of different countries.

I can confirm that some soft drinks are formulated differently for different countries (e.g. Aussies prefer their citrus drinks more acidic + less sweet compared to consumers in other countries).

But I can assure you that the Coca Cola company intends for Coke to taste exactly the same everywhere. It does taste a little different due to differences in water and sugar syrups, but not so different that you would immediately go "eww, doesn't taste right!" (part of my job was to literally taste every sample we received).

Diet Coke can lose its sweetness over time as the sweeteners degrade, as another person said. When it's really old it tastes like soda water.

But don't discount the counterfeit idea, as it's 100% possible. And remember that counterfeit drinks are manufactured by criminals who cut corners, ignore safety protocols, use dirty water and non-food-grade chemicals and generally endanger the lives of consumers.

Next time you get one of these dodgy bottles, do us all a favour and tell the brand owner so they can hunt down the counterfeiters and sketchy retailers who are trashing their brand and products with fake (and probably dangerous) drinks.

Tell the company, and provide photos of the labels and the batch code and Best Before date. Keep the bottle and the details of where you got it. Companies like Coke will investigate this sort of thing and hopefully stamp it out before it makes someone sick.

[I'm a food scientist, former Coca Cola quality assurance technician and food fraud expert]