A woman's menstrual cycle affects the severity of respiratory symptoms, potentially worsening conditions such as asthma, a study suggests.
Norwegian researchers studied almost 4,000 women, and found worse symptoms around ovulation.
Writing in the American Journal of Respiratory and Critical Care Medicine, they said it may be possible to adapt women's medication.
Asthma UK said it could help women with asthma manage their condition better.
All the women studied had regular menstrual cycles lasting 28 days or less, and none were taking hormonal contraceptives.
Of those studied, 28.5% were smokers and 8% had been diagnosed with asthma.
Wheezing symptoms were worse between days 10 to 22 of cycles, with a slight dip near the point of ovulation for most.
Shortness of breath was worse on days seven to 21, again with a slight fall around ovulation.
The study found it was not just women diagnosed with asthma who experienced these symptoms and variations.
Coughing was worse following ovulation for those with asthma, those who were overweight and smokers.
When an individual woman has her period is determined by complex hormonal processes over the course of her cycle.
Throughout, levels of different hormones rise and fall - and body temperature rises around ovulation.
The researchers suggest that these fluctuations may have direct effects on airways. and indirect effects on inflammatory responses to infection.
Writing in the journal, the researchers led by Dr Ferenc Macsali, of the Haukeland University Hospital in Bergen, Norway, said: "We found that respiratory symptoms varied significantly during the menstrual cycle.
"There were large changes in symptom incidence through the cycle for all symptoms."
They also found "pronounced" symptom variations during the menstrual cycle in women with asthma, and say the findings suggest women might need tailored medication regimes.
"Adjustment of asthma medication to the menstrual cycle may potentially improve the efficacy of asthma treatment and reduce disability and health costs related to asthma in women."
Dr Macsali added: "Our results point to the potential for individualising therapy for respiratory diseases according to individual symptom patterns.
"Adjusting asthma medication, for example, according to a woman's menstrual cycle might improve its efficacy and help reduce disability and the costs of care."
Dr Samantha Walker, of Asthma UK, said: "This research is really interesting, and could help women with asthma to manage their condition better.
"Asthma can be triggered by many different things, and this varies from person to person - but we always encourage people with asthma to be aware of things that trigger their symptoms so that they can take steps to control them.
"If women with asthma notice that their symptoms are worsening at key times of the month then they can take preventive measures such as having inhalers that are within date, working and contain enough doses of medicine to see them through the times when they are most affected."