Through a last-minute legislative maneuver on May 7, the U.S. Senate blocked a measure that would have allowed American consumers to purchase prescription drugs from abroad.

Afterwards, Senator Sanders of Vermont described the maneuver as a “poison pill” for drug importation. Oddly enough, Mr. Sanders’s flip comment hits at the very heart of this issue. Despite the fact that many lawmakers have been advocating drug importation for years, it is precisely the threat of poisonous, counterfeit, and unregulated drugs flooding America that has kept such legislation from passing.

Consider, for example, the widespread notion that drug importation would simply open our medicine cabinets to shipments from countries like Canada, Great Britain, and France. Such an impression couldn’t be further from the truth.

Because of open trade between the European Union’s 27 member states, drugs purchased from Britain or France could have originated in Latvia, Malta, Cyprus, or elsewhere in the EU.

Further, most large Internet pharmacies in Canada admit to getting their drugs from Europe. So drugs purchased from our northern neighbor could have easily started off in those countries, as well. This raises serious safety concerns.

The EU doesn’t require drug importers to record the batch numbers of parallel importations, so if a particular batch of medicines originally intended for sale in Greece is recalled, tracing where the entire batch has gone is impossible. Elsewhere in the EU, it’s not uncommon to find misstated dosage strength, tablets errantly replaced with capsules, and mislabeled expiration dates.

Also, the world’s drug market is awash in counterfeits. According to the World Health Organization, as much as 10% of today’s global medicine supply chain is counterfeit. And the U.S. Food and Drug Administration has reported that in some countries, that figure jumps to 50% or higher.

Moreover, terrorists have already proven that they are adept at exploiting weakened chains of pharmaceutical custody. According to a recent report from the federal Joint Terrorism Task Force, a global terrorist ring with ties to Hezbollah is importing counterfeit drugs into America by way of Canada.

But not to worry, say supporters of drug importation. Any legislation that passes, they promise, will include a clause demanding the FDA ensure that imported drugs are safe, legal, and properly labeled. Simply demanding such a guarantee, however, hardly assures that the FDA actually has the ability to follow through with it.

Finally, drug importation leaves our nation open to the serious health risks posed by an unregulated drug marketplace. Because of Canada’s Drug and Pharmacies Regulation Act, it is illegal for a Canadian pharmacy to dispense a prescription drug without the written authorization of a Canadian doctor. To skirt the law, Internet pharmacies pay Canadian doctors to sign prescriptions for American patients. This is a recipe for disaster.

A few years ago, for instance, a Toronto-based physician was found guilty of co-signing 24,212 prescriptions over six-and-a-half months so that American citizens could have their prescriptions filled by an Ontario pharmacy. Considering that, on average, this one physician signed more than 124 prescriptions each day, it’s obvious that he didn’t seriously evaluate the merits of each script he signed.

And whether it’s a teenager who hopes to mix Viagra, Ritalin, and vodka for the party of his life or a senior citizen who mistakenly orders the wrong dosage of Lipitor, the consequences of such renegade action can be deadly.

Despite these facts, drug importation isn’t dead. The AARP and its congressional allies continue to claim that drug importation is a surefire way to give seniors and other consumers immediate access to cheaper drugs. In the coming weeks, the House of Representatives is expected to consider the issue.

Policymakers must confront the very serious business of ensuring that drugs entering our markets are legitimate and safe. Such behavior might not score political points, but it will make certain that we don’t open our borders to an uncontrollable influx of untested, impure, expired, and counterfeit drugs from around the world.

Mr. Pitts is president of the Center for Medicine in the Public Interest and a former FDA Associate Commissioner.