The morning after pill is not 100% effective
Giving women their own stash of morning-after pill to keep "just in case" does not work, findings suggest.
A review of international data by the UK-based Cochrane Group found advance provision of emergency contraception had no impact on pregnancy rates.
But neither did it increase the chance of a woman having unprotected sex or catching a sexually transmitted infection.
The researchers say the policy fails if women choose not to take the pills.
Lead investigator Chelsea Polis said: "Some women may not use emergency contraception when needed, even if they have it in advance.
"Like condoms, emergency contraception will not work if it is not used."
But she agrees that women should still be given information and "easy access" to the morning-after pill.
"It is a safe and effective way to prevent unintended pregnancies for individual women who will use it when needed."
Women who take the morning-after pill within the first 72 hours after unprotected sex are less likely to become pregnant.
Emergency contraception has been available to buy over the counter at pharmacies in the UK for nearly a decade. GPs can also prescribe it.
But some say it should be made even easier to access.
It can be difficult to arrange an appointment with a doctor on public holidays or weekends, for example.
One proposal is to give women a set of pills to keep at home that they can use should the need arise.
But the Cochrane review, which included 11 trials and involved over 7,500 women from the US, China, India and Sweden, found this policy did not alter pregnancy rates.
Norman Wells of the Family Education Trust said: "This new review is in line with earlier studies which have consistently shown that the advance supply of the morning-after pill doesn't make the slightest difference to unintended pregnancy and abortion rates."
He says making emergency contraception more readily available sends out the wrong message and gives a green light to sexual activity.
"The easy availability of the morning-after pill is part of a mix that is lulling young people in particular into a false sense of security and encouraging a more casual attitude to sex."
Rebecca Findlay of the Family Planning Association said the study showed women could be trusted to use emergency contraception responsibly when they have it in advance.
"And it doesn't, as a minority wrongly suggest, encourage them to take risks and have unprotected sex.
"It's essential that women of all reproductive ages can have emergency contraception in advance if they want it.
"The quicker you take it the more effective it is, and we can see from this review that women who did have it at home took it 13 hours earlier than those who didn't."