You have to pick:

Since you're unable to tell the truth, you could use that to gain knowledge of the universe just by playing 20 questions. I think people would overlook my annoying habit of always lying if I fixed the worlds problems.

That is an interesting interpretation, although I would describe call what you are describing as "Be unable to say anything true". In other words, "telling the truth" is an expression that means "not lying". It DOESN'T necessarily mean saying what is absolutely true.

Suppose A is true but you think that A is false. I think it would be "telling the truth" to say that A is false, and lying (aka not "telling the truth") to say it is true. Even though it is in fact true in an absolute sense.

So I don't think this would give you any information about stuff you don't already know (or think you know).

Anyway, that is a lot of yammering about a minor point. I just don't want this option to artificially inflated in value.

Surprisingly, this isn't a particularly bad option.

Metallic mercury is not very reactive, it'll just pass right through your system (it was used as a laxative by ancient Romans). It's only once mercury has formed organomercury compounds that it gets really toxic and eats your brain.

Methanol is somewhat toxic but has easy and effective treatments to prevent it from harming you. Amusingly, one of the easiest treatments is to just drink a whole lot of ethanol as well. Methanol itself isn't the real problem, it's the toxic byproducts that are produced when the liver metabolizes it. By drinking a whole lot of ethanol you can keep your liver occupied metabolizing that instead, until all the methanol has left your body by direct means (sweat and urine).

Disclaimer: Don't rely on the internet for medical advice. If you've drunk a glass of methanol and mercury in real life call the local poison hotline immediately. I am not a doctor and I'm mostly just rambling from memory.