Such cryoprotectants don't exist... yet
Currently, it is not possible for a human to survive being frozen (or a vacuum for that matter). The expanding substances in our bodies ruptures cell walls killing us on a cellular basis. The cryoprotectants you would need will need to be a fictional substance that would fortify human cell walls in a way that allows a human to survive being both frozen and subjected to a vacuum. Because this technology does not exist yet, it means that this drug can take on any form you so choose. When/if anyone ever discovers a workable cryoprotectant, maybe it will be something you can eat, maybe not, but until then, it is writer's choice. Since the drug could come in any form, then any way you could consume anything would be a possible delivery method; so, just make up what ever scenario you find most interesting.
Easiest way to accidently take it
Cryoprotectants will most likely be handled as a medical resource, just like we let doctors handle vaccines, radiation pills, antidotes, and other drugs that can help us prepare for hostile conditions. So whatever the cryoprotectant is, it will most likely be stored under lock and key in the ship's medical facilities. This means that accidently getting this medicine in your food is highly unlikely.
However, what does sometimes happen is that a doctor will administer the WRONG medicine. So, it's not that your protagonist was not trying to ingest some medicine, it's just that some overworked or under experienced pharmacist grabbed the wrong thing from the cabinet. So when you're poor protagonist isn't feeling well and goes to the infirmary for some Bismuth Subsalicylate, instead he gets a nice dose of cryoprotectants. On the down side, he does not feel any better after taking his medicine, but I guess getting to survive the vacuum of space in the next chapter is a fair trade-off.