In fact, poppy-seed-related positives are such a legitimate concern that the federal Substance Abuse and Mental Health Services Administration actually increased the positive reading threshold for opiate drug tests from 300 nanograms (ng) per milliliter of urine to 2,000 nanograms per milliliter in 1998 (for government workplace drug tests) to lessen the risk of false positives.
It's hard to say exactly how many poppy seeds you would have to eat to test positive, since factors like body weight vary from person to person. But according to a 1998 study published in the Forensic Science International journal, eating two poppy-seed rolls with an average of about .76 grams of seeds each caused positive test results for up to six hours in one individual, ranging from 47.9 ng/ml to 832 ng/ml, depending on how long it had been since ingestion. And poppy-seed cake? The same study found that eating a slice (which contained an average of 4.69 grams of seeds) caused all four subjects to screen positive for up to 24 hours—four times as long as the rolls—citing one individual's results ranging between 83.8 ng/ml and 302.1 ng/ml.
But don't relax and binge on poppy-seed muffins: "Different testing agencies require different levels of certification," says Whiteman, noting that some even have zero-tolerance policies. The choice of cutoff is the client's, she says, and many choose the lower government option of 300 ng/ml, such as all of Norchem's clients that test people on probation in Texas. So, to be safe, if you need to take a drug test—in Texas or elsewhere—skip the poppy seeds.