How often do people fail drug test without using?

24% A false positive is defined as a drug free sample falsely being reported as positive for drugs. This can occur for a number of reasons including: improper laboratory procedure, mixing up specimen samples, incorrect paperwork, and passive inhalation.

The following have been reported to cause a false positive for barbiturates:

BARBITURATES
Prescription Meds:
Fiorinal for tension headaches, Donnatal for the treatment of irritable bowel syndrome and stomach ulcers, and antiepileptic medications containing phenobarbital (i.e. Dilantin)

Prescription NSAIDs:
Anaprox, Tolectin, ifenoprofen, fluriprofen, oxaprozin, Ansaid, Clinoril, Dolobid, Feldene, Indocin, Lodine, Meclomen, Nalfon, Naprosyn, Orudis, Relafen, Voltaren

Others: Kidney infection, kidney disease, diabetes, liver disease

Many over the counter cold medicines or decongestants and other meds contain compounds such as ephedrine, pseudoephedrine, phenylpropanolamine, phenylhexadrine, etc. that are structurally related chemically to the more powerful amphetamines. These compounds usually have minor stimulant actions. They often cause false positive reactions for amphetamines in drug test.

It's not that poppy seeds contain an opiate-like substance that causes false positive for opiates in drug screens but are contaminated with opium from the seed pods. The seed pods are the source of opium.