"Side Effects and Warnings
Tea tree oil taken by mouth is associated with potentially severe reactions, even when used in small quantities. Several reports describe people using tea tree oil by mouth who developed severe rash, reduced immune system function, abdominal pain, diarrhea, lethargy, drowsiness, inflammation of the corners of the mouth, slow or uneven walking, confusion, or coma. There have also been reports of nausea, unpleasant taste, burning sensation, and bad breath associated with tea tree oil use. Many tea tree preparations contain large volumes of alcohol.

"… Animal research suggests that tea tree oil used on the skin in large quantities can cause serious reactions such as difficulty walking, weakness, muscle tremor, slowing of brain function, and poor coordination. …"

All these are talking about using it in mouthwash or in a product that is applied all over the body. But if you’re allergic (or think you are) you probably can become more sensitive the more you use it.

I might suggest you try another product for cold sores that contains L-lysine, known to help reduce the duration and reccurance. If you get them frequently, also take L-lysine internally. Available at health food stores. Read about one popular brand here: http://www.quantumhealth.com/productgroups/coldsore.html