Mefenamic acid is a nonsteroidal anti-inflammatory drug (NSAID), which is the name for medicines that are used to treat mild-to-moderate pain and inflammation. Mefenamic acid tablets are usually taken to provide relief from period pain (dysmenorrhoea), reduce the symptoms of heavy periods (menorrhagia), and treat migraines caused by periods.

Mefenamic acid relieves period pain by blocking the production of prostaglandins, which are hormones released by your womb before and during your period. Prostaglandins cause your womb to contract, and if too many are released then these contractions can become painful. Mefenamic acid causes fewer prostaglandins to be released.

Heavy periods, like period pain, can happen when your womb releases too much of the hormone prostaglandin. Mefenamic acid blocks the production of this hormone, causing your periods to become lighter. Mefenamic acid has been shown to reduce heavy bleeding by between 20 to 50%.

Mefenamic acid takes between 2 - 4 hours to kick in and start easing your period pain. Each 500mg dose of mefenamic acid kills pain for up to 8 hours, so you may need to take it two or three times a day for all-day relief.

Mefenamic acid tablets are swallowed whole with water. Take 500mg of mefenamic acid on the first day of your period, and stop once your period becomes lighter or your pain becomes manageable. You can take 500mg up to three times a day, usually for 2 to 4 days.

It’s a good idea to eat something before taking mefenamic acid, as it could give you an upset stomach if you take these tablets on an empty stomach.

Do not take more than 1500mg in a single day (24 hours), as there is a risk of seizures. You also shouldn’t take other NSAIDs, like ibuprofen, aspirin, or naproxen, while you’re taking mefenamic acid as it could damage the lining of your stomach and intestines.

If you’re unsure whether it’s okay to take a certain medication while taking mefenamic acid, have a chat with your pharmacist or message one of our doctors through your patient account.

If you’ve only just missed your dose, then it’s fine to take it when you remember and take your next dose as normal. If it’s almost time for your next dose then you shouldn’t take a double dose. Just skip the missed dose and take your next one as normal.

It’s really important not to take an extra dose to make up for a missed dose. Taking too much mefenamic acid in a short period of time can increase the chances of side effects or overdose.

When used as instructed by a doctor, mefenamic acid will generally cause little-to-no side effects. If you have any of the below side effects while taking mefenamic acid, you should stop taking it and have a chat with your GP before continuing treatment.

Common side effects of mefenamic acid include:

• stomach pain
• feeling sick
• bowel problems, like constipation or diarrhoea
• feeling dizzy and/or sleepy
• a ringing sound in your ears (tinnitus)
• headache 
• heartburn 

Mefenamic acid can also cause some more serious side effects, which require urgent medical attention. If you start getting any of the below side effects after taking mefenamic acid, stop taking them and go to A&E as soon as possible

• vomiting
• breathing difficulties
• swelling of the skin or rash
• chest pain

For more information, read the patient information leaflet.

You shouldn’t take mefenamic acid while you are breastfeeding. While there aren’t many studies into the effect of mefenamic acid on breastmilk, the medicine could potentially make your breastmilk unsafe for your baby to drink. Other NSAIDs, such as ibuprofen, are recommended because they are proven to be safe.

Yes, it’s totally fine to take your contraceptive pill while taking mefenamic acid. However, combined contraceptives (which contain both progesterone and oestrogen) can help with painful and heavy periods, so you may want to try a combined contraceptive pill as treatment if you’re not already.