This week, Arena Pharmaceuticals (Nasdaq: ARNA) announced its application to market its obesity drug lorcaserin in the EU. While I have no idea how that might justify the 35% move we saw in the stock in the two days following, it does seem like lorcaserin might have a slightly better chance at approval there.

Lorcaserin, VIVUS' (Nasdaq: VVUS) Qnexa, and Orexigen's (Nasdaq: OREX) Contrave were all rejected by the Food and Drug Administration because the U.S. agency's hard-line stance on the balance of risks and benefits for obesity drugs. When diet and exercise are an option, the bar is set fairly high, although second-round approvals, at least for Qnexa, are looking a little more likely.

In the EU, companies have had a much easier time gaining approval for drugs; the two agencies are usually looking at the exact same data, so the only logical explanation is that the European Medicines Agency has a lower threshold for risk-benefit analysis than the FDA.

Even with the seemingly easier pathway to approval, there's still a big risk that the EMA doesn't approve lorcaserin or the other obesity drugs. The EMA seems to be taking just as hard a stance as the FDA when it comes to heart issues with obesity drugs. In fact, Abbott Labs' (NYSE: ABT) Meridia was pulled off the market in Europe months before the FDA took action.

If the obesity drugs were able to get on the market, they could have an easier time getting to patients, though. Obesity is a long-term health concern, which means helping someone shed pounds now only helps a U.S. insurer if the patient keeps the same insurance for an extended period of time. Since people switch insurance frequently, there's less incentive to cover obesity drugs or assign them a low-tier co-pay to encourage their use.

The European health-care system, on the other hand, is fairly holistic since there's often only one government insurer covering most of the patients. The European health-care systems tend to be fairly price conscious, but they're also forward-thinking; spending money now on obesity drugs could save a lifetime of diabetes or heart drugs in the future.

If you're going to include potential EU sales in your valuation model, I'd suggest applying a high risk adjustment, and discount them by at least 50% (more if you're trying to be more conservative). Without an advisory panel system like in the U.S., investors won't get much insight into what the agency is thinking until the decision is made.

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Fool contributor Brian Orelli holds no position in any company mentioned. Click here to see his holdings and a short bio. The Motley Fool owns shares of Abbott Laboratories. Motley Fool newsletter services have recommended buying shares of Abbott Laboratories. The Motley Fool has a disclosure policy. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. Try any of our Foolish newsletter services free for 30 days.