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I noticed an old bottle of milk of magnesia in my refrigerator and saw that it had an expiration date printed: January 2015. Checking the ingredients, I saw only three: magnesium hydroxide, purified water, sodium hypochlorite. I understand that milk of magnesia is more or less a mineral solution. How could this require an expiration date? What would be the consequences of taking milk of magnesia past the expiration date?

(I see that this page discusses expiry dates on chemicals in the abstract; I don't know which of these arguments apply to milk of magnesia in particular, or any other over-the-counter dietary supplement or drug. As an IT professional, I have absolutely no experience or expertise in the physical or life sciences.)

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    $\begingroup$ Has nothing to do with chemistry: Laws in many countries require almost anything sold as edible to have an expiry date. Even white sugar. $\endgroup$ – Karl Sep 13 '15 at 23:56
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    $\begingroup$ And as for the magnesia milk: It will keep good as long as there is enough hypochlorite in it to kill off anything that tries to live in there. Keep the lid tightly on and that is forever. If you trust that lid. ;-) $\endgroup$ – Karl Sep 14 '15 at 1:03
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    $\begingroup$ Milk of magnesia is not a solution, but a suspension. Over time, the suspension may segregate; also, the solid phase (being slightly soluble) may recrystallize into larger grains. This is yet another possible reason for the product to have an expiration date. On a side note, once I was surprised to see the expiration date on table salt (NaCl). Can someone think of a chemical reason behind it? $\endgroup$ – Ivan Neretin Sep 14 '15 at 7:47
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    $\begingroup$ I had seen expiry date on calcium carbonate tablets as well. But I generally don't care about it. $\endgroup$ – WYSIWYG Sep 14 '15 at 8:39
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    $\begingroup$ @KarlRatzsch You did indirectly answer the question already, why not copy-edit it into an answer and liberate the comments section? $\endgroup$ – Martin - マーチン Sep 14 '15 at 12:54
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Many countries require anything marketed as edible to have an expiry date. You can speculate about what might spoil your Magnesia (see comments above), but likely it will be good four or fourty times as long as indicated.

Of course the lid might not be perfectly tight, and it could dry up or get contaminated by bacteria, but this can happen anytime. And certainly after some time it will separate into clear water and a thick suspension of magnesia particles and you'll have to shake or stir it very thoroughly before consummation.

Anyway these expiry dates are 99% baloney. Very few things really get bad before you see the mould grow on them.

Fun fact and (german) exemption to that rule: After six months at the latest, the taste of beer (the non-pasteurised stuff) starts to deteriorate. Yet, according to the date on the bottle, you have 12 more weeks to drink the remainder of that crate with increasing disgust before it "expires".

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    $\begingroup$ Expiry dates also helps companies avoid getting sued by someone who felt unresistable desire to consume something obviously spoiled. No one wants to be responsible to some 50 years old table salt. $\endgroup$ – Greg Sep 14 '15 at 22:15
  • $\begingroup$ Even in the US at least higher courts still believe in the concept of responsible adults and educated consumers. You don't actually get those millions for dryfrying your cat in the microwave. ;-) $\endgroup$ – Karl Sep 14 '15 at 22:34
  • $\begingroup$ I wasn't speaking about US exclusively. Or you meant the US is a unique exception? Anyway, I think we can agree on that companies have tendencies to extremely undershoot realistic expiry dates, often 1-2 order of magnitude. Doing so makes no significant cost to them: no one wants sell 50 years old SPAM in their shop, and most accounting system write such down as loss. Trashing old, unsold stocks is actually cheaper than keeping them around. Making our costumers trashing their old stock is not a cost for the company neither. On the other hand, if anything happens, they can loose a lot. $\endgroup$ – Greg Sep 14 '15 at 22:45
  • $\begingroup$ If you actually tried to sell 50 year old spam, you'll be liable for your customers upturned stomachs, with or without an expiry date on the can. If a stupid customer finds it in his basement in 2045, it will make no difference either. $\endgroup$ – Karl Sep 14 '15 at 23:04

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