WASHINGTON • For the first time, the US Food and Drug Administration has approved a digital pill - a medication embedded with a sensor that can tell doctors whether, and when, patients take their medicine.

The approval, announced late on Monday, marks a significant advance in the growing field of digital devices designed to monitor medicine-taking and to address the expensive, longstanding problem that millions of patients do not take drugs as prescribed.

Experts estimate that so-called non-adherence or non-compliance to medication costs about US$100 billion (S$136 billion) a year, much of it because patients get sicker and need additional treatment or hospitalisation.

"When patients don't adhere to lifestyle or medications that are prescribed for them, there are really substantive consequences that are bad for the patient and very costly," said Dr William Shrank, chief medical officer of the health plan division at the University of Pittsburgh Medical Centre.

Dr Ameet Sarpatwari, an instructor in medicine at Harvard Medical School, said the digital pill "has the potential to improve public health", especially for patients who want to take their medication but forget. But, he added, "if used improperly, it could foster more mistrust instead of trust".

Patients who agree to take the digital medication, a version of the anti-psychotic Abilify, can sign consent forms allowing their doctors and up to four other people, including family members, to receive electronic data showing the date and time pills are ingested.

A smartphone app will let them block recipients any time they change their mind. Although voluntary, the technology is still likely to prompt questions about privacy and whether patients might feel pressure to take medication in a form their doctors can monitor.

NYTIMES