Oh what a difference a few weeks can make. As recently as the end of June, shares of Wyeth (NYSE: WYE) were trading at a 52-week high, up nearly 22% year to date. Fast-forward a month, and shares have hit a brick wall, pulling back 22% in the opposite direction. They now rest barely above the stock's 52-week low. What caused this drastic change? And what should shareholders of Wyeth expect in the upcoming weeks and months?

The fall
In late July, an FDA ruling told the company to provide more supporting data on Pristiq, a drug that Wyeth hopes to launch for menopause symptoms. The FDA wants the drugmaker to conduct a long-term study demonstrating that Pristiq does not cause serious cardiovascular or liver problems. Wyeth also seeks to get the drug approved for treating depression, possibly to become the successor to its top-selling antidepressant, Effexor XR.

In recent quarters, Wyeth has also faced the threat posed by generic forms of its existing products. Presently, the company has its hands tied in a legal battle with Teva Pharmaceuticals (NASDAQ:TEVA) in an effort to prevent its launch of a generic form of heartburn drug Protonix, the source of 10% of Wyeth's Q2 net revenue. Wyeth also had previously blamed generics for its Q1 drop in sales of Effexor XR.

Potential upside?
Over the long run, the recent dip in shares of Wyeth could prove an ideal buying opportunity for investors looking to add a large-cap pharmaceutical company to their portfolio. Last week's overall market plunge has presented many buying opportunities for value-oriented investors, with Wyeth only one of several candidates.

Wyeth has remained proactive in tackling its problems with Pristiq and generics alike. Regarding a clinical trial for Pristiq in relation to menopause symptoms, Wyeth does not expect this decision to affect the FDA's review of the drug as a treatment for depression, and it's anticipating a favorable ruling in Q1 of 2008.

In the battle against generics, Wyeth was still able to persevere in Q2, with a 6% increase in sales of Effexor. In the battle over Protonix, Wyeth has sought a preliminary injunction against Teva, maintaining that Teva has violated the patent for the drug's active ingredient, which is not due to expire until mid-2010.

The downward trend in the price of Wyeth shares may have painted a grim picture for the stock's future; however, the company has already demonstrated its ability to grow revenues despite the competition from generics. It still has a few cards up its sleeve; on Monday it launched Lybrel, a birth-control pill, and it expects to launch six other new products over the next year and a half. This movement should afford the company even greater diversity in its portfolio of drugs, and will make the company as a whole even less dependent on the fortunes of any one drug.

In short, calmer seas just might be closer than you think.

Check out Tom and David Gardner's market-beating Stock Advisor newsletter service to see which stocks could give your portfolio a boost. You can see all of the recommendations -- including the latest ones -- with a 30-day free trial.

Fool contributor Billy Fisher does not own shares of any of the companies mentioned. The Fool's disclosure policy is the cure for what ails you.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.