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
9 Upon admission, the patientâs routine immunoassay for amphetamines showed a negative result, but after 3 days, a repeat analysis was positive for amphetamine. A confirmatory test was ordered and identified IV phenylephrine as a likely cause of the false positive.Another drug that has been associated with false positive results for amphetamine is 1,3 dimethylamylamine (DMAA). DMAA has sympathomimetic activity and is an ingredient in some dietary and weight-loss supplements. Upon admission, the patientâs routine immunoassay for amphetamines showed a negative result, but after 3 days, a repeat analysis was positive for amphetamine. A confirmatory test was ordered and identified IV phenylephrine as a likely cause of the false positive.