First, I am only a prenursing student. But I am heavily involve in NAMI (National Alliance for the Mentally Ill) and I have a few friends who have paranoia schizophrenia. So my comments are totally nonprofessional, ok?
As you know, you can't convince them the food or drink is not poison. If you try, it gets even worst without the meds.
A few things you could try that are low risk (in terms of not deepening the paranoia) but no gurantee, a shot in the dark:
- Don't dismiss his fear of being poison. Acknowlege his emotion (fear I am assuming). Ask if there is someway you can help. Maybe you can be his food taster if he wants. If he agrees to it, you can take a drink and take some bite of his food (he choose) first. Now he can always say you have an antidote.
- Another possibility is to ask if there are anything you can do that is within your capacity to make sure there are no poison (you have to think about this one as it might have a risk of deepening his paranoia because he might interpret it as you agreeing that the food are poison). Let's see... maybe something else... how about saying something like as far as you know, the food and drink are not poison but you are willing within reason to do a few extra things for him so he can be sure (like you being a food taster, or heck, cook things himself, is that possibilt?).
As for sneaking meds into food, I agree with previous posters, that is really dangerous. Like you said, lots of them actually can tell. They may be mentally ill, but they are not stupid.
Bottom, he really needs the meds (which is the problem). As the meds weaken the paranoia, the above suggestions will have greater chance of suceeding.
In the mean time, see how much trust you can build if possible. Remember do not think logic, think emotions. That is, see if you can somehow address the emotions behind the paranoia and hopefully that can build some trust.
Oh, here is another possibility (you have to be creative here). You might say something like "it must be horrible to feel that the food or drink is poison." or "I am sorry you think the drink is poison, it must feel awful". They listen and maybe nonchalantly take something from his food and eat it yourself.