LONDON (THE GUARDIAN) - A contraceptive pill that needs to be taken only once a month has been developed by scientists.

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

Scientists said it could help 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 anti-retroviral 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 said the approach could add to the existing range of women's contraceptive options.

But Professor 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 people could swallow that could last for any length of time to treat diseases, like mental health ailments, opioid addiction, Alzheimer's and Aids," he added.

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 per cent 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.

While less than one woman in 100 is 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.

Swallowing a monthly pill, the team said, could reduce such errors in use, potentially reducing the 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, a consultant in gynaecology and reproductive medicine, welcomed the study. "The concept of a monthly oral contraceptive pill is attractive and has the potential to broaden contraceptive choice," she said.

She added there are already many options for women who are looking for an alternative to a daily pill, including a copper IUD (intrauterine device) 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 with around nine in 100 women taking the pill," she noted.