What substances would give a false positive as amphetamines on a drug screen?

If no illegal drugs were used then its a false positive. Some prescription meds have amphetamine i.e. Adderall. If you aren't on any ADHD meds then it is most likely from the otc sinus meds, and/or acid reducers ranitidine zantac can/probably will cause a urine test to show positive also.