Complex sentence: In extremely rare cases, repeated improper topical application of hydroquinone has been associated with exogenous ochronosis and most hydroquinone preparations contain the preservative sodium metabisulfite, which has been linked to rare cases of allergic reactions including anaphylaxis and severe asthma exacerbations in susceptible people.
Split into simpler sentences: In extremely rare cases, repeated improper topical application of hydroquinone has been associated with exogenous ochronosis. Of note, many hydroquinone preparations contain the preservative sodium metabisulfite, which has been linked to rare cases of allergic reactions including anaphylaxis and severe asthma exacerbations in susceptible people.