Hello, Welcome to Just Answer and thanks for your question. What kind of drug test was it - a dipstick-type screening test, or was it sent to the lab for testing via GC/MS?
Screening tests are always supposed to be sent to the lab for confirmation testing via GC/MS because there are so many things that cause false positives for any number of "forbidden" drugs. Insist that they send a backup sample for testing, since a screening test isn't admissible for any kind of legal procedure.
The antibiotic amoxicillin is the only thing I've ever seen cause a false-positive for cocaine on a screening test. Most prescription pain medications, some cough medicines, Nyquil, and various antibiotics can cause false positives for various opiates including methadone. Sertraline (Zoloft) is known for causing false-positives in a screening test.
With GC/MS testing, there are no "false-positives." Every chemical has its own "fingerprint" so that no two will be confused for one another.
Here is a good source that you can take to your testing agency when you tell them that you want backup GC/MS testing. They should be well aware of these facts since they are standard through the medical testing field.