When you see an expired prescription, your first thought is to toss the bottle in the trash, right? However, a new laboratory report reveals that some prescriptions that have expired nearly 4 decades ago may be just as potent as when they were first produced.

Health expert Lee Cantrell, director of the San Diego division of the California Poison Control System and his colleagues, tested eight drugs that had expired 28 to 40 years ago. The drugs contained different active ingredients such as aspirin, codeine and hydrocodone.

The laboratory analysis reported the usual shelf life for drugs ranges between one and five years after production. However, it is a random date set by many companies because the U.S. Food and Drug Administration does not require manufacturers to discover how long a medication retains its potency.

Aspirin and amphetamine, two of the tested ingredients, were found to fall below the 90 percent threshold. Conversely, other ingredients were found in amounts greater than 110 percent of label strength.

"All [the expiration date] means from the manufacturers' standpoint is that they're willing to guarantee the potency and efficacy for the drug for that long," Cantrell said. "It has nothing to do with the actual shelf life."

With consumers spending more than $300 billion a year on prescription drugs, if companies were forced to do longer-term stability tests it can be cost-friendly for consumers, especially in the countries affected by drug shortages, according to Cantrell.

Before you start popping expired medications, keep in mind the drugs that were tested were unopened and still in their original containers. Medications that have been opened or stored in non-ideal conditions may differ. Humidity, temperature and exposure to light can affect how well a drug can withstand the test of time, according to Mohammad Nutan, an associate professor of pharmaceutical sciences at the Texas A&M Health Science Center, in Kingsville.

In addition, health experts warn taking depleted drugs can cause complications.

The study was published in the Archives of Internal Medicine.