Cold sores are caused by the herpes simplex type 1 virus and are contagious whereas canker sores are not. Also, cold sores can appear outside the mouth in places such as under the nose, on the lips or under the chin while canker sores always appear inside the mouth.anker sores, or aphthous stomatitis, are small ulcers in your mouth that cause pain and discomfort when talking or eating. Canker sores come in two varieties: simple and complex.
Codeine, which is found in many pain relievers, may cause a false positive for morphine or heroin because of its similar chemical structure. Antibiotics. Certain newly developed antibiotics including amoxicillin and ampicillin have been reported to cause false positives for cocaine. DHEA
Dental/Medical treatments: Cain-based products, especially Novocaine, have been known to cause false positives for cocaine on a urine drug test and other drug tests as well.Some legal products actually contain small amounts of illegal chemicals.ational laboratories state that the EMIT test is wrong 25% to 30% of the time, and can result in false during a urine drug test positives from legal substances up to 38% of the time!