Historically, the male body was treated as the universal medical baseline, and women of "child-bearing potential" were systematically excluded from early-phase clinical trials, notably after a 1977 FDA mandate. This historical exclusion created a significant knowledge gap regarding how diseases and treatments uniquely affect women.

Drug Metabolism and Adverse Reactions Biological sex significantly alters drug pharmacokinetics—how a drug is absorbed, distributed, metabolized, and excreted. Men and women differ in body composition (such as fat and total water percentages) and liver enzyme activity (such as Cytochrome P450 variants), which drastically affects drug clearance. Because standard dosages were historically optimized for men, women experience adverse drug reactions (ADRs) nearly twice as often as men. A prime example is the sleep medication Ambien (zolpidem); the FDA eventually had to slash the recommended starting dose by half for women because their slower metabolism of the drug resulted in dangerous next-morning impairment.

Disease Manifestation and Epigenetics Sex also dictates how major diseases present at a molecular and physiological level:

• Cardiovascular Disease (CVD): While men typically develop obstructive macrovascular disease and Heart Failure with Reduced Ejection Fraction (HFrEF), women more frequently suffer from coronary microvascular dysfunction and Heart Failure with Preserved Ejection Fraction (HFpEF). Consequently, women often experience what are considered "atypical" heart attack symptoms, leading to under-treatment.
• Autoimmune Diseases: Conditions like systemic lupus erythematosus (SLE) and multiple sclerosis heavily skew female. This is strongly linked to X-chromosome biology. Females have two X chromosomes, and while one is typically silenced (X-inactivation), certain immune-regulating genes like Kdm6a and TLR7 can "escape" this inactivation. This gives women a "double dose" of these genes, heightening immune responses and autoimmunity risks.
• Alzheimer’s Disease: The APOE4 allele, the strongest genetic risk factor for late-onset Alzheimer's, exerts a much stronger pathological effect on women than men, greatly increasing their risk for accelerated cognitive decline.

Regulatory Changes To correct these historical blind spots, the NIH and FDA now enforce policies requiring researchers to evaluate "Sex as a Biological Variable" (SABV) in basic and preclinical studies. They also mandate the inclusion of women in clinical trials alongside transparent, sex-disaggregated data reporting to ensure treatments are safe and effective for everyone.