WASHINGTON - The US Defence Department has advised service members to avoid eating poppy seeds because officials are concerned they could cause misleading drug test results.

The warning reflects long-standing concerns that people who eat a poppy seed bagel or lemon poppy seed loaf sometimes have drug tests come back positive for codeine or morphine, even if they are not using the drugs.

The Defence Department’s under-secretary for personnel and readiness, Mr Gilbert Cisneros Jr, issued a memorandum last week directing military departments to advise service members to avoid foods and baked goods containing the seeds out of an “abundance of caution”.

Mr Cisneros acknowledged that the concerns about poppy seeds were “not new”. He said the department’s thresholds for a positive drug test aimed to distinguish morphine- and codeine-use from poppy seed ingestion.

But recent data had suggested that some poppy seed varieties contained a level of codeine contamination that made it more difficult to identify whether a positive drug test result was from drug use, Mr Cisneros added.

Poppy seeds do not contain opiates, but when they are harvested, they can be contaminated by the morphine, codeine and thebaine that are in a fluid that oozes from the plant.

How much of these opiates wind up on seeds sold in grocery stores or in muffins bought at bakeries vary widely, researchers said.

Ms Michelle Carlin, an assistant professor of toxicology and forensic chemistry at Rutgers University, said she had analysed bags of poppy seeds from different grocery stores and found that “even within the same bag, there’s a big variation in the amount of morphine, codeine and thebaine”.

A variety of factors affect these levels, including where in the field the poppies were grown and how much sunshine and water was in that area, Ms Carlin said.

To minimise opiate contamination, seeds are supposed to be washed and processed, but that does not always happen. Some seeds are also sold unwashed for people seeking illicit access to opiates without a prescription.

The Food and Drug Administration said in a statement that there was no requirement for poppy seeds to be washed before being sold, but “manufacturers are required to ensure that food is safe for consumption.”

“In general, when poppy seeds come to America, they’re meant to be washed away of all that morphine and codeine,” said Associate Professor Madeleine Swortwood, a forensic science expert from Sam Houston State University. “And several years ago, we found that there were companies that were importing unwashed poppy seeds, and the unwashed poppy seeds then inherently have lots of morphine and codeine on them.”

All these factors mean that “the concentration of these opioids in the poppy seed varies quite a lot,” said Asst Prof Marta Concheiro-Guisan from John Jay College of Criminal Justice.

And that can lead to a positive drug test, even if the bagel or cake that caused the positive result did not affect the consumer.

That poppy seeds can create misleading drug test results has been known for decades. In a 1996 episode of the television sitcom Seinfeld, Elaine Benes lost her job after a positive drug test caused by a poppy seed muffin.

The people who interpret routine drug tests look for a certain ratio of morphine and codeine that typically indicates a positive result is caused by poppy seeds, not drug use. But the Defence Department memorandum noted that “recent data” suggested poppy seeds could show a higher amount of codeine than previously expected.

Asst Prof Concheiro-Guisan said she had also observed this change in recent reports. “It’s getting very tricky,” she said.

For people who are not in the military but have an upcoming drug test, all three experts interviewed said they should definitely avoid poppy seeds for a few days, if not weeks or months, before the test. NYTIMES