Is it dangerous to use too much of oralgel with benzocaine for a canker sore?

Oragel O.D. Orajel is a topical anesthetic with the risk of toxicity and and the rare but serious condition called methemoglobinemia. Therefore, application should not exceed the manufacturer's recommendations. It is also dangerous to use a topical anesthetic for a chronic oral lesion that may be cancerous. Any such lesion should be evaluated and possibly biopsied.
Can cause allergy. In addition to the previous answer, Benzocaine is a frequent contact sensitizer, especially when used locally like that. If you become allergic to it, the allergy may include a lot of related compounds. Best to stay away from Benzocaine in general .
Sometimes. It can be dangerous in young children, especially infants, and in some older children and adults with a rare genetic defect. A recent warning on using teething gels with benzoczine has been issued by the government. For more information got to www.Fda.Gov/medwatch and search on "benzocaine".