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This guy suggests that after you open a bottle with vitamin D supplement, you have just about 2 weeks to consume it before it completely loses its potency. Because when you open the bottle, oxygen gets in and can begin the decay process of the vitamin D into inactive metabolites.
Eur J Hosp Pharm. 2017 Mar; 24(2): 115–119.
Published online 2016 May 6. doi: 10.1136/ejhpharm-2016-000895
Shelf life after opening of prescription medicines and supplements with vitamin D3 for paediatric use
Žane Temova and Robert Roškar