Generic drug manufacturer Teva Pharmaceuticals filed an Abbreviated New Drug Application in 2000 seeking authorization to market the tablets on the grounds that the '252 patent is invalid, unenforceable and/or not infringed. Subsequently, Knoll sued for infringement of the '252 patent, but the District Court for the northern district of Illinois ruled that the patent is invalid on the grounds of obviousness.

On appeal, the Federal Circuit found that although the prior art seems to suggest combining an opioid, such as hydrocodone, with various NSAIDs, there seems to be no record of evidence of teaching or suggesting the enhanced biomedical effect of the combination of hydrocodone and ibuprofen; therefore a genuine factual dispute exists as to the obviousness of the asserted claims. Contrary to the district court's ruling, the Federal Circuit, by majority, would allow Knoll to present evidence of the unexpected benefit of using the hydrocodone/ibuprofen combination, even though it was discovered after the '252 patent had been issued.

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