First off, I am assuming you're talking about L-methionine. This is an 'essential' (meaning we need it and can't make it ourselves) amino acid. I have seen nothing about it being dangerous, excepting for people who are 'poor methylators' (AKA 'MTHFR’ - a methylenetetrahydrofolate reductase deficiency). For those people it leads to high homocysteine which indeed has deleterious health effects.
In humans (no idea about mice) methylenetetrahydrofolate reductase is controlled by two genes, so the population breaks down into quartiles, the bottom one (both genes 'off') being the most affected. Oddly enough, it seems randomly distributed ('oddly' because it seems to offer reproductive benefit).
As someone in one of those middle quartiles, I'd avoid taking a L-methionine supplement unless I was also consuming 'methyl donors' (e.g. betaine, lecithin).
I'd also note that it's used to counter a Tylenol overdose - but that person has much larger and more immediate issues than high homocysteine.
Just my $0.02 - and I'm not a doctor.

The TL;DR of this is that if you don’t know your MTHFR status, you might think about getting a genetic readout — or getting your homocysteine levels checked. (My PCP tells me this test is no longer in the standard-of-care for annual physicals — so you need to ask.) If it’s high, switch to methylated B6, B12 and folate.