When Pfizer (NYSE: PFE) bought King Pharmaceuticals, Pain Therapeutics (Nasdaq: PTIE) and DURECT (Nasdaq: DRRX) inherited a little extra marketing firepower for their development-stage pain drug, Remoxy, which was licensed to King.

But being partnered with big pharma can have its downsides for investors. Because one drug isn't material to the day-to-day business, a pharma company like Pfizer doesn't have to disclose why the Food and Drug Administration rejected a drug.

When the FDA turned down Remoxy at the end of 2008, Pain and King disclosed that the FDA wanted more data but that no additional clinical studies would be needed.

Today, with Pfizer in charge, the press release acknowledged the receipt of a complete response letter and said, "Pfizer is working to evaluate the issues described in the Complete Response Letter and plans to have further discussions with FDA around them."

Real. Helpful. Guys.

Until Pfizer discloses more, investors are left guessing. We know the FDA isn't completely against abuse-resistant pain drugs; it approved Pfizer and Acura Pharmaceuticals' (Nasdaq: ACUR) Oxecta earlier this week. The most obvious possibility has to do with the manufacturing the drug. On its first-quarter conference call last month, Pfizer mentioned that it was working on the manufacturing section of the marketing application.

If it is just something minor, Pfizer might be able to clear it up quickly and get a Class 1 review, which the FDA shoots for responding to within two months. In that case, the substantial haircuts Pain and DURECT took today are certainly unjustified.

But it could be worse. A lot worse. Until Pfizer discloses more information, investors might be better off having Pain and DURECT as part of their watchlist than their portfolio. Just click on the green box next to their tickers, and if you don't have a watchlist, click here to sign up.

Fool contributor Brian Orelli holds no position in any company mentioned. Click here to see his holdings and a short bio. Motley Fool newsletter services have recommended buying shares of Pfizer. Try any of our Foolish newsletter services free for 30 days. 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 Motley Fool has a disclosure policy.