What medication will cause a false positive blood test for methaphetamines?

None, the chemical test for methamphetamine is very specific.

The primary cause of false positives in methamphetamine testing is impure drugs. Almost all Ecstasy (pill form MDMA) is cut with meth, as are much of the lower grades of cocaine available.

addition from another responder-

for a urine test alone, not a gas spectrometer test, even a vicks inhaler can test you positive for meth, as can many over the counter cold medications..those mostly will flag for amphetamines, and many diet aids will.

but if you test positive on a urine test, depending on whos testing you, the GS test is next to verify a fslse or positive reading.