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What: Shares of Clovis Oncology (NASDAQ:CLVS) on Monday have lost 8% of their value, after briefly falling by double digits, following weekend revelations that the company's potential blockbuster lung cancer drug may carry more risks than expected.

So what: Investors had been flooding into Clovis following its announcement that it would present data at the American Society of Clinical Oncology meeting. This presentation took place over the weekend, and media reports indicate that the key takeaway was that some patients in Clovis' phase 1/2 study of CO-1686 must now take medication to counteract high blood sugar caused by the drug. This information was part of an otherwise-positive mound of data, which noted a 58% objective response rate in 40 evaluable patients, of the 81 evaluable patients from whom data was available at the ASCO presentation.

High blood sugar, or hyperglycemia, is typically associated with diabetes, and was observed in 22% of patients taking CO-1686. These patients are taking metformin, which is commonly prescribed to lower blood sugar levels in Type 2 diabetics, but "three or four" patients had hyperglycemia that was severe enough to require regular insulin injections, according to Clovis CEO Pat Mahaffay, who spoke at a company event on Saturday night.

Now what: Clovis has been in a tight race with the much larger AstraZeneca (NYSE:AZN) to develop a similarly targeted lung cancer drug, and these revelations are clearly bad news for Clovis' hopes to outmatch its competitor on efficacy and safety. The drugs are essentially neck and neck in trial efficacy, according to other presentations made at the ASCO meeting, and both have seen adverse effects. At the moment, it appears that neither drug has an indisputable edge, which undermines Clovis' earlier claims to offer a superior treatment to AstraZeneca. Without that edge, Clovis' bull case is significantly weakened, and its future upside is in greater doubt.