Agile Therapeutics (AGRX) - Get Agile Therapeutics, Inc. Report lost approximately half of its value Wednesday despite reporting "positive results" in a study of its Twirla contraceptive patch. 

The Princeton, N.J.-based biotech company released phase three results of the contraceptive study Tuesday after market's close, then saw shares plummet 46% to $2.70 apiece Wednesday. The stock was trading at high volume Wednesday, with nearly 51 million shares swapping hands.

Despite the market's negative reaction, Agile plans to submit its Twirla patch to FDA trials in the first half of this year. 

The poor reaction may be due to investors' interpretation of data on Twirla's effectiveness, which at least on first read indicates the patch might be below the FDA's threshold for efficacy.

In a birth control study, there are two key data points that measure efficacy, analyst Chiara Russo of Cantor Fitzgerald said by phone Wednesday. The Pearl Index, which is a simple math equation that shows the efficacy of the drug, and the upper bound of the 95% confidence interval, which also measures efficacy. 

In the study, the Pearl Index of study subjects 35 years old or younger was 4.80, while the upper-bound of the 95% confidence interval was 6.06.

"The FDA has never approved a combination hormonal contraceptive where the upper bound of the 95% confidence interval around the Pearl Index score exceeded 5.0," analyst Jason Gerberry of Leerink wrote in a note. 

It's no wonder, then, that investors got skittish about the stock. 

Russo noted, however, that part of the problem could have been how Agile discussed the study with shareholders in the past. Throughout discussions of Twirla and its clinical trials, company officials never mentioned a willingness to use overweight patients during its trials. Overweight patients often see less efficacy of birth control. 

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Twenty-five percent of patients who participated in the study were considered overweight but not obese, while 35.3% of those in the study were considered obese. 

According to Russo, had Agile alerted investors to the fact that it planned to use patients with higher BMIs in its study, they would have been more prepared for the study's results. 

The fact that the patch is less effective in overweight and obese patients, while important, may not block the patch's path to FDA approval. 

According to Russo, the FDA places the highest priority on safety of drugs, rather than efficacy.

"Agile has checked the box for safety and study conduct," Russo said. 

The question, though, is how much wiggle room the FDA has surrounding efficacy. In recent approvals, the Pearl Index score and upper bound of the 95% confidence interval have increased as compared to past approvals. As a result, Russo said she still sees a chance that the FDA could approve the Twirla patch. 

"I still think it's a buy, still see a path to approval," Russo said. "It's a much riskier path though."

Russo has a $7 price target on Agile. 

Agile held a conference call after market's close Tuesday.