Poppyseed muffins (or poppy seed muffins) are muffins typically made with wheat flour, butter, sugar and poppy seeds, and often other ingredients (including lemon, cinnamon, vanilla or blueberries). They are often eaten as snacks or as a dessert, but are also commonly eaten as a breakfast.
Americans have been preparing muffins since at least the 19th century, using a wide variety of products to add flavor to the basic batter of flour, sugar and butter (generally not prepared with eggs and by definition unleavened). Fruits and even some garden vegetables have served as flavorings. Poppy seeds were already popular in most parts of the world for their taste and texture—as well as the narcotic characteristics of the opium poppy plant they are harvested from.
In modern times, growing poppy seeds is a difficult business for American farmers, due to the risk of heroin production. Other countries have fewer difficulties with permitting the growth of poppies for the seeds alone, which have very low (but still present) levels of opium alkaloids, such as morphine. As other countries began imitating the American muffin, the occasional use of poppy seeds to flavor them spread as well.
Although poppy seeds cannot be used as a narcotic due to very low levels of opium alkaloids, they do have enough that drug tests are often fooled and give out false positives after an otherwise drug-free person consumes just a few poppyseed muffins. Because of this, all poppyseed pastries place the person who consumes them prior to a test at a high risk of being inaccurately considered a drug user. This defense could, of course, easily be used by someone who had indeed been using heroin or another poppy-based narcotic prior to being tested, and therefore is not generally defensible in court.
Though any poppyseed-based dish (such as curry) could cause such a false positive, in mainstream American culture it is the poppyseed muffin that is the quintessential manifestation of the risk of leading to an inaccurate diagnosis of drug use. From CNN anchors to the Seinfeld episode "The Shower Head", poppyseed muffins are the go-to example of an absolutely benign product giving rise to false drug test results.