Drugmakers' bag of tricks for getting drugs approved may be getting smaller. A new report out from the General Accountability Office says that the Food and Drug Administration is cracking down on non-inferiority clinical trials.

Rather than proving that the drug is better than what's currently available, drugmakers will often run non-inferiority trials that simply try to prove that the drugs work just as well.

The FDA is somewhat right to be worried about non-inferiority trials. If they're run in succession with each drug being compared to the last one that was approved through non-inferiority, there's a chance that the standard could creep in the direction of not working. Imagine if every drug worked 5% less than the previous one. That might be within the statistical margin of error, but would eventually result in drugs that didn't really work.

But non-inferiority trials are very useful for drugmakers and taking them away could hurt their marketing ability. In many instances, a drug that works just as well as drugs already on the market can take market share if it has fewer side effects or an easier mode of delivery. Amgen (Nasdaq: AMGN) used the strategy when testing denosumab against Novartis' (NYSE: NVS) Zometa, but it almost backfired as denosumab has similar side effects. Fortunately for Amgen, denosumab was able to show superiority in two of its trials. In Gilead Sciences' (Nasdaq: GILD) case, non-inferiority was only required because of economics; the company's new quad pill only has to work as well as Atripla because it has to share revenue from the older drug with Bristol-Myers Squibb (NYSE: BMY).

The easy solution is to run both a non-inferiority trial comparing the drug to the current offerings and another against placebo to show the drug actually works. In some cases that'll work: 11 of the 43 drugs that used non-inferiority trials in their marketing applications from 2002 through 2009 used this strategy to get their drugs approved.

But in many diseases, using placebo isn't ethical if there are other treatment options. In those cases, drugmakers may be stuck showing that the drug is superior to what's currently available or hope the FDA is willing to accept only non-inferiority data.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Novartis is a Motley Fool Global Gains selection. The Fool has a disclosure policy.