May impair your reaction skills and affect your ability to remain alert. Do not drive until you know how guaifenesin affects you. Conversely, some people report that guaifenesin keeps them awake. Do not take a dose too late in the afternoon if you experience this effect.

Should I take mucinex before bed?

Expectorants are better for breaking up wet, productive coughs, but don't take them near bedtime.

Will mucinex keep me up?

I found this site searching for "Does Mucinex DM keep you awake?" Yup, it does.”