If a vitamin is expired, can you just double the dosage?

No.

There’s no way for you to determine its current potency. Buy a new bottle.

If it’s expired, all of it is expired – increasing the amount won’t make any difference.

There is a reason why there is an expiration date on vitamins. You cannot know the potency of the medication after it’s expiration date. They don’t lose potency in a linear manner. After a certain time, they are no longer fresh and start to deteriorate, randomly, although you may not know it to look at it. Don’t chance it. Also, don’t flush old pills down the toilet or put them in the trash. Most hospitals, doctor’s offices and pharmacies now have pill takeback programs, or can refer you to one, to keep medications out of the ground water system.

no – expiration dates are arbitrary. something that old would best be tossed.

I think buying a new bottle is better option instead of just double the dosage. We never take the any medicine or vitamins after its expiration date.