MannKind's (Nasdaq: MNKD) investors sure do have a short memory.

Last January, the company said the Food and Drug Administration was delaying its decision on whether to approve MannKind's inhaled insulin, Afrezza. Shares eventually rose 30% in the ensuing two months, before the FDA turned down the drug in March and shares came crashing down.

Nearly a year later, we're in the same situation. MannKind announced today that the FDA will take another four weeks to make a decision about its second attempt at an FDA approval for Afrezza. Investors sent shares up more than 11% at one point today before they fell back.

I can see investors' logic: The delay means the FDA hasn't made up its mind, because otherwise it would have just rejected the drug. But keep in mind that the agency issues a "complete response letter" when it rejects a drug. It isn't a "we looked at most of it, and this part isn't good enough response letter." It's entirely possible that the FDA has already decided to turn down the drug but is just doing its job.

Bulls will point to incidences of new issues cropping up in second attempts at approval as evidence that the FDA issues responses before reviewing everything. Amylin Pharmaceuticals (Nasdaq: AMLN) and its partners, Eli Lilly (NYSE: LLY) and Alkermes (Nasdaq: ALKS), having to run additional heart tests that weren't asked for in the first complete response letter for Bydureon is a prime example.

But it's possible that those new issues were only brought to light in the second review. Maybe a new staffer to the review team pushed harder for them. Perhaps the data in the company's response triggered the added safety concern. Unfortunately, investors aren't privy to much information about regulatory actions, so it's impossible to know.

That isn't to say that MannKind's delay is a negative -- except of course, for the day traders who use investors jumping in before binary events as a way to squeeze out a few percentage points. Without a definitive date, it'll be hard to know when to jump out. Boo hoo.

As I see it, the MannKind delay is essentially neutral. Investors should be cautious about reading more into it than that.

David Gardner thinks you should watch this stock.

Motley Fool Rule Breakers is always on the hunt for hot drug stocks and other cutting-edge picks. See all of our latest discoveries with a free 30-day trial subscription.

Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Fool has a disclosure policy.