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.

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.