The 5 days is simply a guideline, but a good one.
Once opened it is exposed to oxygen and nature simply runs its course.
It’s not necessarily bad for you after 5 days, but obviously it will progressively get worse just like most products do – ie, bananas or milk.
I would suspect it would taste too bad to drink long before it is capable of doing you any harm (the perfect setup for a joke there for someone…)
There are a few things you can do to help extend the life.
The best way to extend the life of alcohol-free wines is to store them in the fridge, even red wines.
This will slow down the process and make the wines last longer.
If you are storing a red wine in the fridge, you’ll want to remove it ahead of time to drink (although there’s no need to leave it too long – contrary to many people’s belief, red wine should be served cooler than modern centrally-heated room temperature anyway).
You can also use something like a Vacuvin Wine Saver to remove air from the bottle first.
Opened bottles of alcoholic wine don’t last long either – it varies on the type but it’s around the same length of time.
