If you have ever been to McDonald’s, you probably know what’s in a Big Mac: as the famous slogan says, “two all-beef patties, special sauce, lettuce, cheese, pickles, onions on a sesame seed bun,” plus a few secret ingredients. But even though this recipe is public knowledge, no one but McDonald’s can make an authentic Big Mac. If another burger maker tried to copy the Big Mac by using the same active ingredients, customers would know the difference.

A similar concept applies to pharmaceutical drugs, and particularly to biological drugs such as the large, complex cancer treatments being used today. Biotech companies may soon be able to produce their own versions of brand-name drugs. But just as it is impossible to make an identical replica of a Big Mac, it is impossible to make an identical replica of a biologic drug.

Congress is considering legislation that would create a regulatory pathway for the approval of “follow-on biologics,” or the mimicked versions of brand-name originator drugs. Much like the generic versions of conventional pharmaceuticals, follow-on biologics have the potential to make certain treatments—in this case, cutting-edge treatments—more affordable.

Just as it is impossible to make an identical replica of a Big Mac, it is impossible to make an identical replica of a biologic drug.

Conventional generics are small, simple molecules—such as the pills we get from the pharmacy—that can be copied identically. Generic Prozac, for example, is chemically the same as brand-name Prozac. It is made using the same ingredients and often the same manufacturing process. The generic drug has the exact same therapeutic effects as the originator. Therefore, generic manufacturers correctly bill their drugs as cheaper alternatives to the name brand. Both drugs are the same active chemical.

Unfortunately, because biologic drugs are so complex, they cannot be copied identically. Biologics are grown in genetically modified living entities, such as plant or mammalian cells. While conventional drugs are made of only a few atoms, biologics are made of thousands. The size of the cancer biologic Rituxan, for instance, is almost 380 times that of the conventional allergy drug Claritin. Even biologic molecules with the same chemical formula can feature different structures throughout their vast array of chemical units. That’s why scientists call mimicked biologics follow-ons—because they “follow on” the originator drugs, but are not generics.

Given the differences between biologic drugs and their follow-on versions, the latter can induce unexpected, unpredictable reactions. Sometimes, follow-on biologics are simply less effective. Other times, they can cause serious adverse side effects. Several years ago in Europe, a follow-on biologic caused patients to suffer from “pure red cell aplasia,” which prevents the body from making red blood cells. Several people died, and many were permanently injured. The allergic reaction not only made patients extremely ill; it also made them allergic to all other forms of the drug, including the natural form produced by their own bodies.

Because of these risks, it is crucial that patients and doctors know exactly which biologic is being used. The easiest way to do that is to require that each follow-on form be identified specifically by name. As federal lawmakers consider a framework for regulating follow-on biologics, they should insist on distinct names for originator drugs and follow-ons. Such a commonsense provision would increase patient safety enormously. Without such an identification system, we will not know exactly where a particular follow-on biologic came from or what version of the drug it represents. Adverse side effects from the follow-on will be impossible to trace quickly, if at all, and patients taking the problematic form of the drug will be harmed. Policymakers should take these risks seriously when deciding on the appropriate regulations.

Bryan A. Liang is executive director of the Institute of Health Law Studies at California Western School of Law.

Image by Darren Wamboldt/The Bergman Group.