Guaifenesin is an over-the-counter oral medication that's used to relieve chest congestion. Most people experience few to no side effects after taking this medication as directed. Guaifenesin comes in different formulations, so you can choose the one that works best for you. It's also available as lower-cost generics.

Common side effects of guaifenesin and phenylephrine may include: dry mouth, nose, or throat; upset stomach, loss of appetite, vomiting; feeling excited or restless (especially in children); sleep problems ( insomnia ); or. headache, dizziness. This is not a complete list of side effects and others may occur. Call your doctor for medical advice.

What is dextromethorphan, guaifenesin, and phenylephrine? What should I discuss with my healthcare provider before taking this medicine? How should I take this medicine? What happens if I miss a dose? What happens if I overdose? What should I avoid while taking this medicine? What are the possible side effects of this medicine?

For regular (short-acting) oral dosage forms (capsules, oral solution, syrup, or tablets): For cough: Adults—200 to 400 milligrams (mg) every four hours. Children 6 to 12 years of age—100 to 200 mg every four hours. Children 4 to 6 years of age—50 to 100 mg every four hours. Children and infants up to 4 years of age—Use is not recommended .