Eli Lilly (NYSE: LLY) had a day it would rather forget yesterday after shelving development of its phase 3 Alzheimer's disease drug, semagacestat, but tomorrow could be just as painful.

The drugmaker will be in front of a Food and Drug Administration advisory panel in its quest to get Cymbalta approved to treat chronic pain. It's an epic quest considering the drugmaker first submitted its application in November 2008.

Cymbalta is already approved to treat anxiety, depression, diabetic nerve pain, and fibromyalgia, but the additional indication could add substantially to sales, which surpassed $1.6 billion in the first half of this year.

Reading through the FDA briefing documents sent to the panel of outside experts, the agency seems to have two issues with expanding Cymbalta's use. First, there's some contention about the way the efficacy was determined. Eli Lilly included patients who later dropped out of the study, which could add to the positive findings even though the patients might have dropped out because they couldn't tolerate the medication.

More importantly, there's the issue of safety. Sure, Cymbalta is already on the market, but there's a big difference between treating diabetic nerve pain or fibromyalgia and treating osteoarthritis and lower back pain where current treatments exist. Cymbalta has the potential to cause liver damage and has a black-box warning about suicide in children and young adults. With other options available -- NSAIDs such as Pfizer's (NYSE: PFE) Celebrex and stronger painkillers like Johnson & Johnson's (NYSE: JNJ) Tylenol 3 and Purdue Pharma's Oxycontin -- the advisory panel is going to be very careful about recommending approving a drug with a less-than-stellar safety record.

Which way will the advisory panel vote and will the FDA follow the advice from the panel of experts? It's hard to predict because a risk-benefit analysis is so subjective. In the case of VIVUS' (Nasdaq: VVUS) obesity drug, Qnexa, the advisory panel fell on the side of risk. For Valeant Pharmaceuticals (NYSE: VRX) and GlaxoSmithKline's (NYSE: GSK) epilepsy drug, Potiga, the panel landed on the benefit side despite the potential side effects.

Eli Lilly investors sure hope this panel will accept the side effects while voting in favor of the benefits. They could use something to deaden the pain right about now.

Fool Michael Olsen thinks this business model has serious potential to lose you money.

Pfizer is a Motley Fool Inside Value pick. GlaxoSmithKline is a Global Gains choice. Johnson & Johnson is an Income Investor selection. Motley Fool Options has recommended a diagonal call position on Johnson & Johnson. Try any of our Foolish newsletter services free for 30 days

Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The Fool owns shares of GlaxoSmithKline and has a disclosure policy.