@spider109 I manage the evening dose by taking mucinex 1200mg with my last glass of water after dinner, which is at least 3+ hours before bed. As suggested elsewhere, I also eat some carbs (pretzel, bread, crackers) with the glass of water to keep the water from sloshing around and sneaking up my esophagus. Sometimes there's some overlap of mucinex doses. In other words, my doses are not always separated by 12 hours. So far this has not been an issue for me. We often go for evening walks which I think helps digestion.

@blm1007blm1007 Guaifenesin/mucinex is primarily an expectorant, meaning it helps loosen and thin mucus in the airways, making it easier to cough up. From my reading, it does not have an effect on the lower esophageal sphincter.

I had to look up methylxanthine -- caffeine, theophylline, and theobromine. I know that theophylline is an old school treatment for asthma, maybe why we don't see it much anymore. Every morning I need a dose of coffee (caffeine) and every now and then doses of chocolate (theobromine) but I make sure to have early-ish in the day for the reasons you state.