The biomedical community stands at a crossroads between the paternalistic assumptions of the 1970s and the emerging paradigm of Precision Medicine. Before now, it might be considered that we viewed women as members of our society requiring protection from research rather than beneficiaries of it, often invoking foetal safety as justification for exclusion [1][2]. In contrast, the recent push toward individualised therapeutics recognises that biological sex is a fundamental variable shaping pharmacokinetics, pharmacodynamics, and disease expression [3][4]. Precision Medicine cannot progress on a male‑centric evidence base; it requires data that reflect the full diversity of human biology. The shift is not merely ethical, it is scientific necessity.
Historical Background
The roots of women’s exclusion from clinical trials lie in the regulatory response to the thalidomide tragedy of the late 1950s and early 1960s [5]. Thalidomide, widely used in Europe and Canada as a sedative and antiemetic, caused severe limb deformities in thousands of infants exposed in utero [5][6]. Although the United States avoided widespread harm due to FDA reviewer Frances Kelsey’s refusal to approve the drug [6], the global catastrophe profoundly shaped regulatory culture. Women of childbearing potential came to be viewed as inherently ‘vulnerable,’ and foetal protection became a dominant regulatory consideration [1][2].
The 1977 FDA guideline recommending the exclusion of women of childbearing potential from Phase I and early Phase II trials, even if they used contraception or had no intention of becoming pregnant [1]. The policy was sweeping, effectively removing most premenopausal women from early drug development. The rationale was framed as safety, but the effect was a systematic erasure of sex‑specific clinical trial data [1][7]. As the Office of Research on Women’s Health later documented, the policy was interpreted so broadly that it excluded even women whose partners were vasectomised [1].
The result was a generation of therapeutics developed and dosed based primarily on male physiology. Women were not treated as autonomous decision‑makers capable of informed consent; they were treated as potential vessels for foetal risk [2]. This paternalistic framing shaped decades of biomedical research.
The Legislative Shift
By the late 1980s and early 1990s, advocacy groups, congressional leaders, and scientific critics began challenging the exclusionary model [7][8]. Reports from the Government Accountability Office (GAO) revealed that NIH’s early inclusion guidelines were applied inconsistently and enforced rarely [8].
The turning point came with the 1993 NIH Revitalization Act, which mandated the inclusion of women and minorities in all NIH‑funded clinical research unless a clear scientific justification for exclusion was provided [9]. Crucially, the Act required that trials be designed to permit valid sex‑specific analyses, not merely token inclusion [9]. This legislative shift reversed the logic of the 1977 FDA ban: instead of protecting women by excluding them, the new framework protected women by ensuring they were represented in the evidence base.
The Act also catalysed the establishment and expansion of the Office of Research on Women’s Health (ORWH), which became instrumental in promoting sex‑based analysis and monitoring compliance [1]. The Revitalization Act marked the beginning of a structural correction to decades of male‑centric research.
Scientific Insights: The Zolpidem Case Study
The importance of sex‑specific data became dramatically clear more recently with the FDA’s decision to halve the recommended dose of zolpidem (Ambien) for women [10]. Post‑marketing data revealed that women metabolised the drug more slowly than men, resulting in higher morning blood concentrations and impairing of next‑day performance in. various ways, including risk of motor vehicle accidents [10][11].
Although zolpidem had been on the market since the early 1990s, the sex difference in clearance was not adequately characterised during development, an omission directly traceable to the historical exclusion of women from early‑phase trials [3][10]. The FDA’s 2014 safety communication acknowledged that women experience significantly higher serum levels at equivalent doses, necessitating a sex‑specific dosing recommendation [10].
This case became a watershed moment: a widely used drug, prescribed to millions, had been dosed for decades without accounting for fundamental biological differences [3]. It underscored that sex‑specific analysis is not a matter of fairness but of patient safety. The zolpidem example is now frequently cited in policy discussions as evidence that Precision Medicine cannot succeed without sex‑disaggregated data.
Current 2015 Challenges
Despite major policy advances, several barriers to equitable inclusion persist in 2015:
Physician ‘Triage’ Bias
Studies throughout the 1990s and 2000s documented that clinicians often act as informal gatekeepers, selectively referring or recruiting patients based on assumptions about compliance, risk, or reproductive status [12]. Women, particularly those with caregiving responsibilities, are often presumed less available or less reliable participants [12]. This ‘triage bias’ undermines formal inclusion policies and perpetuates disparities in enrolment.
Lack of Data on Pregnant Women
Pregnant women remain one of the most understudied populations in medicine [13]. The legacy of thalidomide and diethylstilbestrol (DES) continues to shape regulatory caution, resulting in a near‑total absence of controlled clinical data for pregnant participants [13]. As Merkatz notes, the DES tragedy reinforced the perception that pregnancy research is uniquely perilous, leading to decades of avoidance [2].
The consequence is that most medications used during pregnancy are prescribed with limited or no supporting data from randomised trials [13]. This gap is not ethically neutral: withholding research shifts risk from controlled studies to uncontrolled clinical practice.
Logistical Burdens: Childcare, Transportation, and Time
Women, especially those with caregiving roles, face structural barriers to participation [14]. Clinical trial schedules often conflict with work and childcare responsibilities. Transportation challenges, lack of compensation for time, and rigid visit requirements disproportionately affect women [14]. These burdens are rarely addressed in trial design, despite being well‑documented contributors to under‑enrolment.
Persistent Male‑Normative Research Culture
Even with mandates in place, many preclinical studies continue to rely predominantly on male animals and cell lines [4]. As late as 2014, analyses showed that sex was often unreported or uncontrolled in basic research, limiting the translational value of findings [4]. The NIH’s 2014 policy requiring consideration of sex as a biological variable in preclinical research represents progress, but cultural change remains slow.
Conclusion
From where we stand now, the trajectory of women’s inclusion in clinical trials reflects a profound shift, from a paternalistic model that sought to protect women by excluding them, to a Precision Medicine framework that recognises sex as a critical determinant of safety and efficacy [3][4]. The transition moves beyond distributive justice (ensuring fair access) toward biological necessity (ensuring accurate dosing and risk assessment).
Perhaps we are seeing signs that the male medical model that dominated the late 20th century is giving way to a more inclusive, data‑driven approach to research. But these are early days. True precision requires not only policy mandates but also cultural change, structural support for participation, and a commitment to generating evidence that reflects the full spectrum of human biology.
References
- Office of Research on Women’s Health. History of women’s participation in clinical research. National Institutes of Health; 2014.
- Merkatz RB. Women in clinical trials: an introduction. Food Drug Law J. 1993;48(2):161–174.
- Soldin OP, Mattison DR. Sex differences in pharmacokinetics and pharmacodynamics. Clin Pharmacokinet. 2009;48(3):143–157.
- Clayton JA, Collins FS. Policy: NIH to balance sex in cell and animal studies. Nature. 2014;509:282–283.
- McBride WG. Thalidomide and congenital abnormalities. Lancet. 1961;278(7216):1358.
- Carpenter D. Reputation and Power: Organizational Image and Pharmaceutical Regulation at the FDA. Princeton University Press; 2010.
- Mastroianni AC, Faden R, Federman D. Women and Health Research: Ethical and Legal Issues of Including Women in Clinical Studies. National Academies Press; 1994.
- Taylor HA. Inclusion of women, minorities, and children in clinical trials: opinions of research ethics board administrators. J Empir Res Hum Res Ethics. 2009 Jun;4(2):65-73.
- National Institutes of Health. NIH Revitalization Act of 1993. Public Law 103-43.
- S. Food and Drug Administration. FDA Drug Safety Communication: zolpidem dosing recommendations. 2014.
- Greenblatt DJ, Harmatz JS, Singh NN, Steinberg F, Roth T, Moline ML, Harris SC, Kapil RP. Gender differences in pharmacokinetics and pharmacodynamics of zolpidem following sublingual administration. J Clin Pharmacol. 2014 Mar;54(3):282-90.
- Bartlett C, et al. The causes and effects of socio-demographic exclusions from clinical trials. Health Technol Assess. 2005;9(38):1–152.
- Shields KE, Lyerly AD. Exclusion of pregnant women from clinical research. Clin Pharmacol Ther. 2013;93(6):499–501.
- Lovato LC, et al. Recruitment for controlled clinical trials: literature summary and annotated bibliography. Control Clin Trials. 1997;18(4):328–352.
