InterMune's (ITMN:Nasdaq - news - commentary - research - analysis) shares fell 20% Tuesday because its top-selling drug, Actimmune, now appears to be less effective than previously thought, according to follow-up data released by the company Tuesday.

In August, the Brisbane, Calif.-based company released results from a 330-patient study of Actimmune that it said proved the drug to be the first and only successful treatment for patients who suffer from idiopathic pulmonary fibrosis, a fatal disorder that causes lungs to fill with scar tissue.

InterMune continued to follow these patients for another three to five months. The analysis of that data was released Tuesday and it shows that the previously reported survival advantage for patients taking Actimmune has decreased, compared with patients who took a placebo.

These results have fired up the already-heated debate over Actimmune's efficacy and its revenue potential. Doctors who treat IPF patients, for the most part, have said that they would increase their use of Actimmune based on the original study results. These doctors, when asked, said they were less focused on actual numbers from the study. Instead, they were happy to see a drug, for the first time, show an ability to prolong the lives of their terminally ill patients.

Whether that sanguine outlook remains once these new findings are digested is still an open question, and one that obviously has investors nervous. InterMune shares closed Tuesday down $5.41, or 19.6% to $22.21. More than 7.5 million shares traded hands, or more than eight times the average trading volume.

InterMune released the new data in conjunction with its appearance at the J.P. Morgan H&Q Healthcare conference. While expressing disappointment that the follow-on data weren't stronger, InterMune executives -- addressing an overflowing breakout room -- insisted that Actimmune still provides a clear benefit to patients with IPF, especially since doctors have no other good medical options.

According to the follow-up data released Tuesday, the overall survival advantage in the Actimmune-treated patients compared with placebo patients fell to 25% from 40%. The difference is not statistically significant, but then, the original data were also not statistically significant.

InterMune, however, was basing most of its original claims of efficacy on a much stronger analysis of a sub-group of patients in the study who had mild to moderate forms of idiopathic pulmonary fibrosis. But here, too, the new data appears weaker.