Who should not take guaifenesin?

Do not take guaifenesin if you:
  • have phenylketonuria, or PKU, a genetic condition that causes mental retardation if phenylalanine (a component of guaifenesin) is consumed 
  • are allergic to guaifenesin
  • are pregnant or breastfeeding
In addition, guaifenesin should not be given to a child under four years of age without authorization from a doctor.