Women warned their menstrual cycle can affect how well certain drugs work

Women’s periods could affect how well some medicines work, and scientists are only now starting to understand why.

Hormones naturally rise and fall throughout the menstrual cycle, with estrogen changing significantly between phases and those shifts can affect the body in plenty of ways.

New research suggests they may also influence how women respond to certain drugs and vaccines.

"For too long, the menstrual cycle has been treated as background noise in health research," said Poppy Cooper, an author of one of the recent studies cited by NY Post. "This work is part of a growing shift in that thinking and a recognition that the menstrual cycle has impacts beyond reproductive health."

And it could have implications for some surprisingly common treatments.

Women have been warned that their menstrual cycle can affect how well certain drugs work for them (Getty Stock Images)

COVID vaccines

We already know COVID vaccines can temporarily affect menstrual cycles.

Research found that vaccination was linked to 'small and temporary' changes, with cycles increasing by around half a day on average.

Now researchers have found that when women receive the vaccine during their cycle could matter too.

A June study found that women vaccinated during the follicular phase, which begins on the first day of bleeding, were 35% more likely to experience side effects than those vaccinated during the luteal phase, the period leading up to menstruation.

But there was another striking difference as women vaccinated during the follicular phase avoided infection for around 200 days longer than those vaccinated during the luteal phase.

"This study treats the menstrual cycle as a variable that matters rather than a confounding variable to simply control away," Julia Craggs, a women’s health doctor, told New Scientist.

"We may be sitting on a significant source of unexplained variation in how women respond to treatment, and we have simply never looked in that direction."

'For too long, the menstrual cycle has been treated as background noise in health research' (Getty Stock Images)

A drug that could work differently in women

Another study has put the spotlight on davenutide, a drug originally tested for progressive supranuclear palsy.

The treatment failed in 2012, but when researchers revisited the data more than a decade later, they spotted something that had been missed.

Women appeared to respond to the drug while men did not.

Researchers then tested the drug in mice this year and found that female mice absorbed more of it when estrogen levels were highest.

A small human trial involving two men and six women produced similar results, with women taking up more of the drug than men.

One possible explanation is estrogen’s effect on blood flow around the brain and the blood-brain barrier.

The findings have helped put the drug back into development.

Hormones naturally rise and fall throughout the menstrual cycle, with estrogen changing significantly between phases and those shifts can affect the body in plenty of ways (Getty Stock Images)

And there could be plenty more

A 2025 review found that estrogen can affect CYP enzymes, which help the liver break down medicines and remove substances from the body.

These enzymes process an estimated 50% to 80% of common clinical drugs, meaning the potential impact could be pretty significant.

Among the medicines processed through CYP pathways are ibuprofen, caffeine, omeprazole, alprazolam and sildenafil.

Researchers found that estrogen suppressed two CYP enzymes, CYP1A2 and CYP2C19, potentially making it harder for certain drugs to be metabolised.