Thanks to Mythbusters, we know that eating a couple poppy seed-laced bagels really can make you look like a heroin fiend, at least as far as a standard drug test is concerned. That makes sense. Heroin is derived from morphine, which is itself derived from poppies. So there's a chemical family tree that's fairly easy to follow.
At the I Can Has Science blog, however, they've taken the standard question—"Will poppy seed bagels make me look like a stoner?"—and turned it around. "Can poppy seeds get me stoned?" (From a purely theoretical, chemistry thought-experiment perspective, of course.) The question turns out to be more valid than you might suspect. We're not talking about dried banana peels, here.
Morphine itself can be administered (by licensed professionals) as a prescription painkiller. Though it's more commonly given intravenously, it is also available orally as a tablet. A standard oral dose consists of about 10 to 30 mg of morphine. Is it possible to get a full dose of morphine from eating poppy seeds?
One report in the Journal of Forensic Sciences found that the morphine content of poppy seeds varies widely with poppy seed source. Spanish poppy seeds seem to have the most morphine - about 251 micrograms of morphine per gram of seeds. This translates to about 0.025% morphine by weight. Thus, to get a medically relevant dose of morphine (10 mg) from Spanish poppy seeds you would have to consume ... About 40 grams of poppy seeds!
It seems like a lot, but how hard would that actually be? A standard baking conversion for dry ingredients is about 8 grams per tablespoon, and one poppy seed bagel probably has, what - a teaspoon or two? By that math, you'd probably have to eat around a dozen poppy seed bagels all at once. However, this delicious looking cake recipe calls for an entire cup of poppy seeds, or approximately equal to 128 grams! Granted, the recipe yields 10 to 12 servings, but one would only have to eat 3 or 4 slices of this Bundt cake to get up to a full prescription dose of morphine! (If the most potent poppy seeds were used.)