GlaxoSmithKline's (NYSE:GSK) biggest problem? It's at the front end of the patent cliff that almost all drugmakers will face. Pfizer (NYSE:PFE), Bristol-Myers Squibb (NYSE:BMY), and Eli Lilly (NYSE:LLY) will see major drops in revenue in a few years as their blockbusters start to face generic competition, but Glaxo is facing that problem now.

The best reason to invest in Glaxo now? It's at the front end of the patent cliff. Bear with me for a moment. When those other drugmakers are hurting a few years down the road, Glaxo should be sitting pretty, seeing the fruits of all those partnerships it's investing in now. You've got to be in it for the long term, of course.

There's no getting around it: the first quarter for Glaxo was ugly. Sales were down 5% at constant currency. Revenue was dragged down by a 22% decline in sales in the U.S. because of generic competition for its epilepsy treatment, Lamictal, and its migraine drug, Imitrex. The stronger dollar helped bring up net sales (recorded in pounds sterling), but the apples to apples comparison wasn't pretty and there's no telling if the dollar will stay at this level or retreat.

Excluding restructuring charges, earnings per share were up 3%, but that was entirely due to foreign sales being boosted by the British pound. If currencies had remained constant, adjusted earnings per share would have come in a whopping 28% lower.

Despite the horrible quarter, there's still plenty to like about Glaxo for the long term. The acquisition of privately owned dermatology specialist Stiefel Laboratories earlier this week continues the tradition of diversification, which, like health-care conglomerate Johnson & Johnson (NYSE:JNJ), should help deaden the blow from generics. As long as enough of its pipeline drugs make it to market, Glaxo should be able to pick itself up, dust itself off, and get growing again once it finishes its fall off the patent cliff.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Pfizer is an Inside Value recommendation. The Fool has a disclosure policy.