Why Sex Matters in Preclinical Research: Importance and Impact

Female biology has long been treated as a complicating factor in medical research rather than a fundamental component of human physiology, according to historical biomedical data. For decades, researchers relied heavily on male cells, male animals, and predominantly male clinical trial populations during drug discovery. This reliance stems from the assumption that male and female biology are sufficiently similar, allowing scientists to use male systems as a default. However, a growing body of evidence demonstrates that biological sex influences almost every stage of disease development and treatment response. As a result, women are nearly twice as likely to experience adverse drug reactions than men, according to historical health data.

The Historical Male Default in Preclinical Research

Male animals have long served as the standard experimental model for modern biomedical research. Female mice were frequently excluded because researchers believed that fluctuations in reproductive hormones during the estrous cycle introduced excessive variability. Scientists argued that this variability made experiments harder to interpret and required larger numbers of animals. Practical considerations, such as the perceived costs of housing both sexes and the desire to simplify experimental design, further reinforced this preference. Over time, this practice became entrenched as scientific convention.

“Females are not simply small male mice,” Jarić told DDN. “Their physiology is different. This means that how drugs are processed and how they respond can be different.”

Despite increasing awareness, male bias remains widespread in preclinical research.

“Every single cell has a sex,” Jarić said.

From Awareness to Implementation in Biomedical Policy

Similar initiatives have since been adopted by the European Commission and the Canadian Institutes of Health Research, while scientific journals like Endocrinology and Physiology require researchers to factor SABV into study design.

These initiatives have produced measurable progress. However, while more researchers include both sexes, many still do not analyze results by sex, indicating a need for better integration of the SABV policy throughout study execution.

Irina Kovlyagina, a neurobiologist at the Johannes Gutenberg University Mainz in Germany, believes the biggest obstacle is no longer reluctance to include females, but a lack of practical expertise. “Most researchers haven’t been trained in sex-inclusive experimental design,” she told DDN.

The network provides practical guidance, training workshops, and evidence-based recommendations. It also hosts an annual symposium covering sex differences in brain health, pain, pharmacology, and metabolic, oncological, and respiratory diseases. The scientific community no longer needs convincing that sex matters; instead, it needs to develop the experimental, statistical, and reporting standards required to study it rigorously.

The Danger and Opportunity of New Approach Methodologies

The shift toward sex-inclusive research coincides with another major transformation in drug discovery: new approach methodologies (NAMs). These technologies, including human organoids, organs-on-chips, and advanced computational models, are increasingly adopted to generate human-relevant data and improve clinical predictions.

If female biology remains underrepresented in the data used to build and validate these models, the blind spots of conventional preclinical research could transfer directly into the next generation of drug discovery tools.

Machine learning models are rapidly becoming powerful tools for identifying therapeutic targets and predicting toxicity, but their performance depends on the quality and diversity of training data. When historical datasets disproportionately represent male biology, algorithms may inadvertently learn and reinforce those biases. This is already occurring in other areas of healthcare. When presented with identical cases differing only in gender, the model was significantly more likely to describe men’s health concerns using terms like “disabled,” “unable,” and “complex,” while equivalent issues in women were omitted or described less seriously.

“These models have enormous potential, but if you feed them biased data, you will get biased outcomes.”

Toward a Complete Picture of Human Biology

Changing policy is only the first step toward representative healthcare, according to researchers in the field. The larger challenge lies in embedding sex-inclusive thinking throughout the entire scientific process, from experimental design and statistical analysis to peer review and scientific training. Without this cultural shift, researchers might comply with reporting requirements without fully considering how sex influences the biology they study.

This necessity extends beyond simply including both sexes in experiments. Researchers must also consider the diversity within female biology. Hormonal changes associated with puberty, the menstrual cycle, oral contraceptive use, pregnancy, lactation, perimenopause, and menopause influence physiology, disease susceptibility, and therapeutic response.

Jarić’s own work highlights how dynamic these biological changes can be. During the normal estrous cycle in mice, researchers observed substantial changes in chromatin accessibility across the brain without any disease or experimental intervention. Other studies show that pregnancy induces structural changes in the brain that persist for years, while investigators increasingly examine how perimenopause and menopause influence neurological health and disease risk. Accounting for sex and life stages from the earliest stages of discovery gives researchers an opportunity to develop safer medicines and build preclinical models on a complete understanding of human biology.