curtis,

hair can be contaminated with cocacine from the outside (by hands…), so in this case cocaine would be on the hair not in the hair, and this could cause a false positive result. In a reliable laboratory, they would be able to differ from cocaine on and in the hair, though. Some sources mention an antibiotic amoxicillin and tonic water to cause false positives, and one laboratory mention “some over-the-counter medications” but it does not mention any specific one.