Synthetic hormones in oral contraceptive pills may be impairing fear-regulating processes in women's brains, scientists have found in a study.

The researchers from Canada-based universities investigated the short- and long-term effects of using combined oral contraceptives (COCs), which contain synthetic hormones, on fear-related brain regions and the neural circuitry which processes fear.

Oral contraceptives are pills that contain female sex hormones, such as oestrogen and progestin, to inhibit ovulation and thus, prevent conception.

The researchers found that compared to men, healthy women currently using COCs had a thinner ventromedial prefrontal cortex, which is a brain region thought to sustain emotion regulation, such as decreasing fear signals when one is in a safe situation.

The prefrontal cortex in the brain lies right behind the forehead and has been studied to play a role in activities such as planning and decision-making.

"Our result may represent a mechanism by which COCs could impair emotion regulation in women," said Alexandra Brouillard, a researcher at the Université du Québec à Montréal and first author of the study published in the journal Frontiers in Endocrinology.

"(It) suggests that COCs may confer a risk factor for emotion regulation deficits during their current use," said Brouillard.

The researchers said, however, that the impacts of COC use may be reversible once intake is discontinued.

They said they thought so because their findings of the effects of COCs on the ventromedial prefrontal cortex were not observed in past users. Thus, they said, the findings would need further corroboration in future studies.

For the study, the researchers recruited 180 participants, aged 23-35 years, in four groups - women currently using COCs; women who used COCs previously but not at the time of the study; women who never used any form of hormonal contraception; and men.

When being prescribed COCs, girls and women are informed of various possible physical side effects such as how the sex hormones could affect their menstrual cycles and ovulation.

However, the effects of these sex hormones on brain development, which can continue into early adulthood, are rarely addressed, the researchers said.

Therefore, they said, given the widespread use of COCs, it was important to better understand their current and long-term effects on brain anatomy and emotional regulation.

Regarding this study, the scientists said that no causal relationship can be implied between COC use and structural changes in the brain and thus, their findings could not be generalised to a wider population just yet.

They also cautioned that drawing conclusions from anatomical findings to behavioural and psychological impacts was not possible at this point.

"Our aim is to increase scientific interest in women's health and raise awareness about early prescription of COCs and brain development, a highly unknown topic," said Brouillard.

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