Sex Differences in Pain Research: What Your Preclinical Program May Be Missing

In 2016, the NIH mandate on sex as a biological variable (SABV) formalized what pain researchers had known for years: males and females process pain differently. Despite this, the majority of preclinical pain studies still rely predominantly — or exclusively — on male animals. The consequence is a systematic blind spot in the data that sponsors carry into clinical development.

The Clinical Reality

Chronic pain conditions disproportionately affect women. Fibromyalgia, migraine, temporomandibular disorders, irritable bowel syndrome, and many neuropathic pain states show female prevalence ratios ranging from 2:1 to 9:1. Women also report higher pain intensity, greater pain-related disability, and different analgesic response profiles compared to men.

These are not subtle differences. They are large enough to influence clinical trial outcomes, endpoint selection, and responder analyses. Yet the preclinical data packages supporting most analgesic INDs are generated almost entirely in male rodents.

What the Preclinical Data Show

When sex is incorporated as a variable in animal pain models, the differences are both measurable and mechanistically informative.

In pig postoperative pain (POP) models, female animals demonstrate higher pain sensitivity than males across multiple endpoints, including von Frey mechanical allodynia and distress behavior scores. Females also show higher intraepidermal nerve fiber (IENF) density, a finding with direct clinical correlates: IENF density is a standard diagnostic biomarker in human neuropathic pain and varies by sex in clinical populations.

These are the first demonstrations of sex differences in postoperative pain in domestic pigs, a translational species whose peripheral neurobiology closely mirrors human. The finding that sex differences manifest not only in behavior but in quantifiable tissue-level biomarkers (IENF density) strengthens the argument that these differences are biologically grounded, not artifacts of behavioral scoring.

Pharmacological responses also diverge by sex. In open field assessments of neuropathic pigs, morphine produces dose-dependent aggressiveness in male animals, a behavioral toxicity signal that is absent or attenuated in females. Buprenorphine shows no analgesic effect in the same model regardless of sex. These drug-by-sex interactions have obvious implications for clinical dosing and safety, yet would be invisible in a male-only preclinical program.

Beyond Pain: Sex Differences in Neurodegeneration Models

The pattern extends beyond pain. In spinal cord injury (SCI) models, female rats recover faster than males on both locomotor (BBB scale) and electrophysiological (transcranial motor evoked potential amplitude) measures. In stroke models, young females show the largest infarct sizes, larger than young males and larger than aged animals of either sex. That finding contradicts the common assumption that stroke outcomes are uniformly worse in older populations.

In global ischemia models mimicking cardiac arrest, diabetic comorbidity eliminates the therapeutic response to neuroprotective agents, but this interaction is only detectable when comorbidity and sex are both included as design variables.

Each of these findings carries translational weight. And each would be missed by the standard approach of testing young, healthy, male rodents.

The Regulatory Direction

The FDA and EMA are paying attention. Regulatory guidance increasingly emphasizes the importance of sex-stratified preclinical data, particularly for CNS and pain indications where sex differences in clinical outcomes are well-documented. Programs that include sex as a variable in preclinical studies are better positioned to anticipate and address regulatory questions about generalizability.

More practically, sex-inclusive preclinical data can inform clinical trial design: stratification strategies, endpoint selection, and dose-finding in ways that improve the probability of success in pivotal trials.

What This Means for Your Program

Incorporating sex as a variable does not require doubling your preclinical budget. It requires designing studies that include both sexes from the outset, powering them appropriately, and selecting models and endpoints that are sensitive to sex-dependent effects.

The data are clear: sex differences in pain and neurodegeneration are real, reproducible, and clinically relevant. The only question is whether your preclinical program is capturing them.

MD Biosciences incorporates sex and diversity variables, including age, sex, and comorbidity, across its pain, SCI, and stroke model platforms. For guidance on sex-inclusive preclinical study design, contact neuro@mdbiosciences.com.