An approval decision on Pozen’s (NASDAQ: POZN) “safer aspirin” has been pushed back three months while the Chapel Hill drug developer conducts a clinical study comparing the two dosages of the drug candidate.

An approval decision on Pozen’s Pozen’s PA8140 and PA32540 was expected by January 24. Pozen said it will now conduct a phase I study comparing the two dosage forms of its drug candidate, which was developed for secondary prevention of cardiovascular disease. The new Food and Drug Administration approval decision date is now April 25. Pozen shares fell 3 percent in mid-day trading to $9.38 on the news.

The delay is not completely unexpected; Pozen disclosed last month in its third quarter earnings release that the FDA requested a teleconference for mid-November. The FDA wanted more clinical data on the effects of PA8140. In a mid-quarter update last month, Pozen CEO John Plachetka declined to speculate on what it would take to address the FDA questions but at that time he suggested it could be done without new trials.

“These NDAs (new drug applications) are extremely voluminous and they have bits and pieces in different places reviewed by different parts of the FDA review team,” Plachetka said. “We think we can put together a valuable summary of this information that can address the information request.”

But Pozen has now determined it will need to conduct another clinical trial to answer the FDA questions. Pozen said that the final data are expected in March.

Pozen’s PA drugs are combination drugs that use Pozen’s delayed release technology to deliver aspirin in a way that reduces the gastrointestinal irritation patients can experience with aspirin alone. Pozen’s PA pills consist of omeprazole, which reduces stomach irritation, in a layer surrounding an aspirin core. PA32540 is the 325 mg dosage; PA8140 is the 81 mg dose.

Both PA dosages contain 40 mg of omeprazole in a form that is immediately released when the tablets are taken. The two doses are manufactured using “nearly identical procedures,” Pozen said. The new study will compare the two drugs with the goal of providing more information about omeprazole after a patient takes the PA tablet. The goal of the phase I study is to study the pharmacokinetic profile – what happens to the drug after it is administered – of the omeprazole part of PA8140 and compare it with the pharmacokinetic profile of the PA32540 tablet. The phase I study will enroll up to 30 patients and cost an estimated $750,000.

The additional clinical trial not only delays an approval decision on the PA drugs, it also delays milestone payments Pozen stands to receive from drug partner Sanofi (NYSE:SNY). In September, Pozen signed a commercialization agreement with Sanofi, a deal that paid Pozen $15 up front for U.S. rights to the drug. The deal also pledges another $20 million in pre-commercialization milestones, which include FDA approval of the drug. Sanofi plans to fold the PA drug into its cardiovascular drug portfolio, which is already led by blockbuster heart drug Plavix.