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
1 Common Vitamins Supplements can cause a false positive test. 2  Riboflavin (B2) can return a false positive test for THC. 3  Many herbs that contain B vitamins should be of concern. 4  Diseases such as diabetes, liver disease, and kidney infections may register as opiates.he outcome is very specific to the individual and/or the situation. 1  Here are some of the more common sources of a false drug test: 2  Pain relievers such as Advil, Uprin, Motrin. 3  Menstrual cramp medications like Midol and Trendar. 4  Ibuprofen is known to cause positive for Marijuana.