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Benzocaine is used in many over-the-counter topical anesthetics to treat conditions such as toothaches and burns. (iStock)

Products containing benzocaine, used to relieve pain for problems ranging from teething to hemorrhoids, will get new labels warning of a rare but potentially serious side-effect.

Health Canada announced this week that companies have been asked to add a statement to their packaging warning of the potential risk of methemoglobinemia in sensitive individuals who use the topical anesthetic.

Methemoglobinemia is a condition in which the blood contains too much methemoglobin, a form of the oxygen-carrying molecule hemoglobin that does not bind properly to oxygen. That means the affected person's tissues may not get enough oxygen.

The labels will include the symptoms of the problem, which include weakness, confusion, headache, difficulty breathing or pale, gray or blue-coloured skin. They will warn people to stop using the product and seek medical attention if any of the symptoms arise.

The new labelling will affect gels, sprays, swabs, liquids and lotions but not lozenges containing benzocaine, since lozenges aren't linked to a risk of methemoglobinemia. 

Most of the products are available over-the-counter, but a few require a prescription. They are used to relieve pain or itching caused by conditions that include teething, sore throats, toothaches, canker sores, mouth or gum irritation, hemorrhoids, burns or insect bites.

Benzocaine is also used in desensitizing "sexual enhancement" products and is also sometimes used by dentists and surgeons to numb the mouth and throat during medical procedures.

Health Canada had issued a warning about the risks of using benzocaine in April 2011, but didn't propose any labelling changes at the time.