Brief Answer:
Your husband will be fine.Get him to ER for safety

Detailed Answer:
Thank you for asking!
Mucinex is an expectorant having active ingredient of guanfacine and extended release tablet(600mg) of just one would not be an overdose or an issue.A person can take two in a day every twelve hours. Also it has been 8 hours already and the half life of mucinex is 1 hour and that means most of it is already gone of the system. Its onset starts in 30 minutes and stays for maximum of 4 to 6 hours. So already 8 hours is not because of this guanfacine.
What you have been reading online made you extra cautious for an innocuous thing and i reassure you there is nothing to worry about.
The fatigue is due to the active metabolite b-(2-methoxyphenoxy) lactic acid resulting from its breakdown in liver and we all know lactic acid does the fatigue to a person so no worries. Once Body clears the lactic acid up the fatigue will be gone.
Dizziness and drowsiness is also not some thing to worry about. expectorants do that. Let him sleep for a while and rest . It will be fine. Now the last symptom you mentioned of rapid heart beat so i want you to know that it is not correlated to the mucinex. If your husband has a history of cardiac issues or palpitations secondary to some deranged cardiac profile, get him evaluated for a cardiac profile by simple single EKG in the ER.If that comes up clear i am sure there is nothing to worry about.
Mucinex these days come in combination with other medicines like phenylalanine and phenylephrine and these are stimulants and they may cause a heartbeat or palpitations.
rest assured the husband is fine, just a clearance of heart beat is needed.Get him to ER and get him evaluated for any trouble which is least likely.It is just to be on the safer road.
I hope it helps. Don't forget to close the discussion please.
Regards
S. XXXXXXX

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