Half the world's population needs to use them for a week each month, every month for about 30 years.
So why are sanitary products - used to absorb menstrual blood and therefore many would argue essential - taxed?
Campaigns have recently been launched in a number of countries demanding the removal of the tax on tampons and sanitary towels.
What is the situation in different countries around the world?
Australia is one of a number of countries where petitions were recently launched on the issue. As it stands, sanitary products attract the 10% Goods and Services tax (GST).
Australian treasurer Joe Hockey had asked for an exemption to be looked at as part of a review of taxes.
But Australia's states and territories, which have to agree to changes made to the GST, decided in August 2015 to keep the tax on sanitary products.
After the UK joined the Common Market in 1973, a 17.5% Value Added Tax (VAT) was introduced on sanitary products. In 2000, after lobbying by Labour MP Dawn Primarolo the tax was reduced to 5%. She explained the reduction was "about fairness, and doing what we can to lower the cost of a necessity".
The issue most recently came up in the general election campaign, with Ukip promising to end the taxation of sanitary products by withdrawing from the EU. As it stands, zero rates are not allowed in the EU.
David Cameron was asked about sanitary taxation by a student at the University of East Anglia. He answered: "This is a long-standing campaign. I have had a look at it in the past. It's quite difficult."
"Some VAT things you can change. Other VAT things, if they're linked to other products, it's quite difficult to do it within the framework of European laws and I can't remember the answer."
HM Revenue and Customs has said: "The application of VAT in the EU, including rates and flexibilities afforded to member states such as the UK, is governed by EU law.
"The UK applies a 5% reduced rate of VAT to the supply of sanitary products. This is the lowest rate possible under EU VAT law."
Elsewhere in the EU, Slovakia is an example of where tax is paid on sanitary products at the basic goods rate of 20%. Slovakian director Diana Fabianova who made the film The Moon Inside You, said: "There are no plans to change the law. When the sexual revolution and feminism was happening in the West, we had a communist regime. And you can still feel that we skipped these important changes. There is no interest from political sides to stand up for these issues."
Sanitary towels, tampons and panty liners are zero-rated for VAT in Ireland, as the rate was in place prior to EU legislation imposing minimum rates.
As in Australia, a recent petition in Canada has led to government support.
"Thank you - on behalf of 72,000 menstruators!", wrote one person in response.
GST was brought in with assurances that the price of sanitary products before tax would be reduced, local media reported, but some argue this has not happened.