A contraceptive pill that needs to be taken only once a month has been developed by scientists.

The gelatine capsule, which has so far only been tested on pigs, dissolves in the stomach to a release a six-armed star-shaped polylmer structure that sits in the stomach for at least three weeks and releases synthetic hormones to prevent pregnancy.

Scientists say it could help to prevent unplanned pregnancies caused by errors in daily pill use.

Similar drug delivery systems have previously been tested on animals by the same team to deliver anti-malarial drugs and HIV antiretroviral therapy, while it has also been tested on humans for other drugs.

However, the new study is the first time the approach has been used to deliver contraceptives and shown to release a drug over such a long period.

Experts say the approach could add to the existing range of women’s contraceptive options. But Prof Robert Langer of the Massachusetts Institute of Technology, a co-author of the study, said the approach could eventually be applied to an even broader range of applications.

“I hope there will be pills that people could swallow that could last for any length of time to treat different diseases, like mental health diseases and opioid addiction, Alzheimer’s, Aids,” he said.

Writing in the Science Translational Medicine journal, Langer and colleagues report how they designed the capsule and polymer system, tweaking the latter’s structure to increase its surface area, monitoring the rate of hormone release, and testing the system to make sure it could withstand the acid conditions of the stomach.

They then tested two forms of the new monthly pill on a total of six female pigs. Instead of testing the ability of these monthly pills to prevent pregnancy the team looked at the level of the synthetic progestogen hormone released. They then compared these with levels for five sows given a single “daily” oral contraceptive pill of the same hormone.

The team found that both forms of the monthly pill showed a slower and more prolonged release of the hormone than the daily pill. At 21 days, pigs given the monthly pill still had levels of the hormone on a par with levels measured within about one day of pigs given a dose of the daily pill.

After 29 days the hormone was still present in their blood. By contrast the hormone was almost completely cleared within two days for pigs given a dose of the daily pill.

While x-ray imaging revealed that for some pigs the polymer structure of the monthly pill began to break down during the month, this did not result in a sharp drop in the hormone levels.

The team say a spinoff company called Lyndra Therapeutics will work on developing the technology. This includes ensuring the hormones are released over the desired timeframe, that the system subsequently leaves the stomach, and dosages are tailored to humans, with the aim of producing a pill containing synthetic oestrogen and progestogen that women can take just once a month.

Oral contraceptive pills are a popular form of birth control. Unlike long-acting methods such as the contraceptive implant, a pill does not require a clinical procedure to administer – something that might be particularly helpful in the developing world where healthcare services are limited.

However, previous research has suggested up to 50% of women using daily oral contraceptive pills miss at least one dose over a three-month interval, potentially leaving them at risk of getting pregnant. “Even when they are intending to take birth control, people forget,” Langer said.

While less than one woman in 100 are expected to become pregnant if a daily pill is taken reliably, in real life missed doses mean that about nine women in 100 will become pregnant while using such contraception.

Swallowing a monthly pill, the team says, could reduce such errors in use, potentially reducing numbers of unplanned pregnancies.

However, as with other forms of hormonal contraception the new pill, while effective, might have unwanted side effects.

Dr Diana Mansour, the vice president of the UK’s Faculty of Sexual and Reproductive Healthcare, welcomed the study. “The concept of a monthly oral contraceptive pill is attractive and has the potential to broaden contraceptive choice,” she said.

Mansour added that there were already many options for women who were looking for an alternative to a daily pill, including the copper IUD and contraceptive implant, both of which last for years before needing to be replaced.

“These are more effective than oral contraception with fewer than one woman in 100 becoming pregnant each year using these longer acting reversible contraceptives, compared to around nine in 100 women taking the pill,” she said.