The system offers doctors an objective way to measure if patients are swallowing their pills on schedule, opening up a new avenue for monitoring medicine compliance that could be applied in other therapeutic areas.

Shares in Otsuka rose 2.5 percent on Tuesday after news of the U.S. Food and Drug Administration (FDA) late on Monday.

The FDA said that being able to track ingestion of medicines prescribed for mental illness may be useful "for some patients", although the ability of the digital pill to improve patient compliance had not been proved.

"The FDA supports the development and use of new technology in prescription drugs and is committed to working with companies to understand how technology might benefit patients and prescribers," said Mitchell Mathis of the FDA's Center for Drug Evaluation and Research.

The system works by sending a message from the pill's sensor to a wearable patch, which then transmits the information to a mobile application so that patients can track the ingestion of the medication on their smartphone.

About the size of a grain of salt, the sensor has no battery or antenna and is activated when it gets wet from stomach juices. That completes a circuit between coatings of copper and magnesium on either side, generating a tiny electric charge.