Did you report the possible side-effect? Depression doesn't seem to be recorded for Mucinex DM, so it might be worth flagging that up with your country's drug regulator, especially considering you know you weren't the only case.
It's my impression they really appreciate reports of unlisted side-effects: bug reports are great for software, but they're even more valuable when they can mean life or death - and when the affected systems are as complex & varied as the human body.
I seem to recall I did, and they said that it wasn't a reported side effect they had heard of before. I reported it after my friend reported it, since there was some corroboration at that point, and I mentioned that. I believe this was a number listed on the package, but honestly I don't recall it all that well, as it was quite some time ago.
It's entirely possible I called the pharmaceutical company, and they may not be incentivized to look into every claim...
If the DM stands for dextromethorphan (an expectorant) it would fit. They warn people on SSRIs to avoid it because of interactions - has some affect on serotonin. It's also usually included in things like Dayquil.
Dextromethorphan is a "dissociative anesthetic" that some people use recreationally. It's a bit of a cliche that kids will sometimes abuse cough syrup because it gives them a mild ketamine-like high. I suppose it's not shocking that it could potentially have a negative psychological interaction.
It looks like it has SSRI-like effects itself. So it also shouldn't be taken by people on older anti-depressants (MAOIs) as it could cause 'serotonin-syndrome' (which can be fatal if not treated quickly).
It's my impression they really appreciate reports of unlisted side-effects: bug reports are great for software, but they're even more valuable when they can mean life or death - and when the affected systems are as complex & varied as the human body.
The US has MedWatch: http://www.fda.gov/Safety/MedWatch/default.htm
The UK has the Yellow Card scheme: https://yellowcard.mhra.gov.uk/
Most other countries should have something similar.