Hello, Thank you for asking your query at DoctorSpring.
Coccaine usually remains in the hair follicle for maybe 4 months or more, but it varies and depends on how frequent you used to have. This will increasingly apply if you haven't got a hair cut since a long time. But, in your case 10 years is very long time and very low chances the cocaine would still remain in your hair follivles.
Amoxicillin and ampicillin are drugs which will give a false positive test for cocaine. I suggest you go for a repeat testing a week after you are done with the course of amoxicillin. Kidney and liver disorders, along with Diabetes have also been known to show up false positives. You can look in if you have any of the disorders by going for a liver function tests and kidney function tests. Hope this helps. Please feel free to ask follow-up questions