Can a cold sore be treated with toothpaste?

No: You would need to use a cold sore medication. There are several good over the counter ones available at your local drug store.

No-Prevention: Canker or cold sore or more specifically aphthous ulcer can be prevented by avoiding a common foam detergent found in most toothpastes called sodium laurel sulfate. Switching to biotene or other toothpaste not containing this ingredient can be the cure in preventing this painful condition.

Need antiviral: Cold sores are viral, you need zovirax (acyclovir) rx ointment, or abreva, etc. And how effective depends on what stage the cold sore is.