Once opened how long does Lactaid milk last?!

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that all depends on how the milk was treated.
If someone drank out of the carton, it can go bad sooner than the best by date.
If it gets left out often, sitting on the table while folks are having breakfast, it can spoil before the date as well.
On the other hand, if it get stuck in the fridge and lives there undisturbed and sealed, it can last longer than even the 7 days past the best by date.
Thats why you use your senses, smell, taste, appearance.

Be careful here. You are flirting with danger by not listening to your carton. 7 days may not seem like a lot to you, but most warnings are there for a reason. I should know, the last time I tested the limits of a warning label I lost. Let my story be a warning to you, and to anyone else who thinks they are smarter than the labels they read.

While I can't provide a number of days the milk will be usable, it ought to outlast the recommended time of freshness. Lactaid has no milk sugar, so there's less in it to spoil. When it finally goes bad, your nose will know.