The maximum recommended adult dose of guaifenesin is a total of 2400mg per day.
[...]
In case of overdose
If you take too much (overdose)
Immediately telephone your doctor or the National Poisons Centre (telephone # *snip*),
or go to accident and emergency at your nearest
hospital, if you think that you or anyone else may have taken too much guaifenesin.
Do this even if there are no signs of discomfort or poisoning. You may need medical
attention.
Overdose with guaifenesin is unlikely to produce toxic effects since its toxicity is low.
Very large doses may cause nausea and vomiting.
Side Effects
Tell your doctor or pharmacist as soon as possible if you do not feel well while you
are using guaifenesin.
All medicines can have side effects. Sometimes they are serious, most of the time
they are not. You may need medical treatment if you get some of the adverse effects.
Ask your doctor or pharmacist to answer any questions you may have.
If you experience any of the following stop using the medicine and seek
immediate medical help:
difficulty in breathing, swelling of the face, neck, tongue or throat (these are
signs of a severe allergic reaction).
Guaifenesin may occasionally cause nausea or vomiting, stomach pain. This usually
only occurs when more medicine is taken than recommended on the packaging.
Other adverse effects not listed above may also occur in some patients. Tell your
doctor if you notice any other effects.
Do not be alarmed by this list of possible adverse effects. You may not experience
any of them.