Why We Need to Stop Ignoring Females in Addiction Research

For decades, a frustrating pattern has played out in addiction neuroscience: females were basically ignored, not even included in the studies. Then, thanks to the NIH SABV mandates almost exactly 10 years ago, researchers all over the country started including females, and things started getting intriguing, with some evidence suggesting that the estrus cycle may influence the motivation to take cocaine.

Our lab just published a paper in Psychopharmacology, led by Dr. Elizabeth Sneddon, that forces us to rethink the relationship between the estrous cycle and cocaine use. What we found surprised us.

The question we started with was simple:

Does the phase of the estrous cycle (the rodent equivalent of the menstrual cycle) influence how much cocaine female rats self-administer? The existing literature would predict yes, that high-estrogen phases should drive greater intake and motivation. So we tested this in our Heterogeneous Stock (HS) rat model, using over 400 female rats, one of the largest female cocaine self-administration datasets ever generated.

The answer: no.

Estrous phase, whether a rat was in proestrus, estrus, metestrus, or diestrus on the day of testing, had no significant association with cocaine intake during short access, long access, or progressive ratio sessions. No difference. Estrous phase crossed with our Addiction Index severity categories? Still no effect of phase.

This is a null finding, and null findings matter. But the story doesn't end there, it gets far more interesting.

When we looked at the cycling patterns themselves, we discovered something unprecedented.

Eighty-two percent of our HS rats showed irregular estrous cycling before they ever saw cocaine. Not mild irregularities, we're talking skipped phases, prolonged durations, atypical sequences. In young adult HS rats sampled every 7-9 hours for five days, 96% were irregular. In adults, it was 100%. By comparison, 60% of our Wistar control rats cycled regularly, right in line with published norms.

This is, to our knowledge, the first report that HS rats exhibit this degree of baseline cycling irregularity. The HS rat is an outbred stock descended from eight inbred founder strains, designed to capture the kind of genetic diversity that actually exists in human populations. It's increasingly used for genome-wide association studies and behavioral phenotyping. And yet nobody had carefully characterized estrous cycling in this model before.

Here's where it connects to addiction vulnerability.

While cycling irregularity wasn't correlated with overall cocaine intake or escalation, rats with the most severe cycling irregularities, those scoring a 3 on our irregularity index, took significantly more cocaine during a session where each infusion carried a 30% chance of being paired with a mild electric footshock. In other words, the rats with the most disrupted hormonal cycling were the ones most willing to endure pain to get cocaine.

Footshock-resistant responding is one of the most translational measures we have in preclinical addiction research. It models the defining feature of severe substance use disorder in humans: continued use despite negative consequences. The fact that pre-existing cycling irregularity predicted this specific behavior, not general intake, not motivation under progressive ratio, but compulsive-like use in the face of punishment is striking.

Why does this matter beyond the lab?

In human populations, 15-25% of women experience menstrual cycle irregularities. Those women show roughly 40% higher rates of mental health disorders, including substance use disorders. But the causal direction is usually assumed to run one way: drugs disrupt hormones. Our data suggest the arrow may also point in the other direction, that pre-existing hormonal dysregulation may shape who becomes most vulnerable to the compulsive features of addiction.

This also has major implications for how we design and interpret preclinical studies. The standard recommendation in the field is to exclude animals with irregular cycling from your dataset. That's understandable from a statistical cleanliness perspective, but it means we've been systematically removing from our studies the very animals that may model the most vulnerable human subpopulation. We can't understand individual differences in addiction if we keep excluding the individuals who differ.

From our lab's perspective, this paper reinforces something we've been building toward for years.

The lab uses HS rats specifically because their genetic diversity produces the kind of behavioral spread you see in human populations: some rats escalate dramatically, some barely change their intake, and everything in between. Our Addiction Index framework captures this spectrum by integrating escalation, motivation, and compulsive-like responding into a single composite score. When we applied it here, the irregular cyclers were overrepresented at both extremes, the most resilient and the most severe. That's not noise. That's a bimodal distribution suggesting hormonal dysregulation may push individuals toward divergent vulnerability trajectories.

This is exactly the kind of biology that gets lost when you use a single inbred strain where every animal is genetically identical. And it's exactly the kind of biology that could eventually lead to genetic mapping of cycling irregularity as a risk factor, something the HS rat platform is uniquely positioned to do.

What we still don't know is a lot.

We don't know the hormonal mechanism driving these irregularities, is it hypothalamic-pituitary-gonadal axis dysregulation? Altered estrogen receptor sensitivity? Something about the genetic architecture of the HS stock? Could it be due to how the HS colony is bred and housed, or a genetic-by-environment interaction? We don't know whether cocaine exposure further disrupts cycling in rats that are already irregular (logistically, we couldn't do week-long cycle monitoring after the escalation phase). And we don't know yet which specific phases, if any, are driving the footshock-resistant phenotype.

These are all questions we can now ask, precisely because we stopped treating irregular cycling as a methodological problem and started treating it as a biological variable worth studying.

The bigger message here is one I keep coming back to in our work:

Addiction does not affect everyone equally, and the reasons why are written in our biology. Sex differences, hormonal variation, genetic diversity, these aren't confounds to be eliminated. They're the map to understanding why one person can try a drug and walk away, while another cannot stop despite losing everything.

Every time we design a study that embraces biological complexity instead of controlling it away, we get closer to treatments that actually work for the people who need them most—including the millions of women whose unique biology has been systematically understudied in addiction research.

I have to be honest about something, though.

We would love nothing more than to continue this line of work and dig into the mechanisms driving female-specific vulnerability to addiction. But last year, the grant supporting this research was canceled by the NIH, then eventually reinstated 6 months later, but the damage was done. The lead author on this paper, Dr. Sneddon, lost faith that the NIH would reliably fund this type of work, and I can't blame her. It's devastating when politics gets in the way of doing good science. Right now, we have no active grant supporting this research program. So I'll say it plainly: if anyone out there, foundations, philanthropists, industry partners, anyone, is interested in funding fundamental research on the biological mechanisms underlying female-specific vulnerability to addiction, please reach out. This work is too important to let die on the vine because of funding instability.

This work was supported by NIDA and the Burroughs Wellcome Fund. Full paper: Sneddon et al. (2026), Psychopharmacology. https://link.springer.com/article/10.1007/s00213-026-07041-8?utm_source=rct_congratemailt&utm_medium=email&utm_campaign=oa_20260324&utm_content=10.1007/s00213-026-07041-8