Excipient (pharmacologically inactive substance)
What is it?
Sodium metabisulfite has a molecular formula of Na2O5S2 and is consider an antioxidant. Sodium metabisulfite is used as a food preservative and in the commercial wine making industry. In the paper and textile industry it is used as a bleaching agent. Sodium metabisulfite can be associated with serious allergic reactions. Although primary exposure in children is through foods, serious reactions have also occurred after oral, inhalational, parenteral, and ophthalmic administration of sulfite-containing drugs. Reactions to sulfites most often occur in patients with reactive airway disease like asthma. Symptoms most frequently reported include wheezing, dyspnea, and chest tightness; anaplylaxis has also been reported. Sulfites have been removed from metered-dose inhalers, including albuterol. The diagnosis of a sulfite sensitivity is made by history and through a challenge testing with a qualified physician. Read food package labels carefully to avoid sulfites in foods. In addition, drug manufacturers are required to list the inclusion of sulfites in medicine product labeling. Metabisulfites may be present in some intravenous preparations. People who do not have a sensitivity to sulfites rarely show any reaction.  
 Napke E, et al. Excipients and additives: hidden hazards in drug products and in product substitution. Can Med Assoc J. 1984 Dec 15; 131(12): 1449–1452. Accessed March 3, 2015 at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1440339/?page=2