New research from the University of Bristol and University of Bath reveals the alarming mechanisms behind why mixing benzodiazepines and opioids proves so deadly. The findings paint a stark picture of a growing public health crisis, with 70% of opioid-related deaths in Scotland also involving benzodiazepines, whilst in England, 93% of drug deaths involve multiple substances, most commonly opioids and benzodiazepines.

The scale of benzodiazepine opioid deaths demands urgent attention. In Scotland, opioids account for around 82% of all drug fatalities. When benzodiazepines enter the equation, the risk multiplies dramatically. The research team conducted laboratory experiments and interviewed 48 people who use both substances alongside healthcare professionals to understand why this combination proves so lethal.

How Benzodiazepines and Opioids Affect Breathing

The laboratory work provides crucial insights into the physical mechanisms behind polydrug overdose fatalities. Using controlled mouse experiments, researchers compared the effects of increasing doses of morphine (heroin metabolises to morphine in the body) with and without diazepam.

The results were striking. When 10 milligrams per kilogram of morphine was administered alone, it decreased the respiratory rate by approximately 30%. However, when just 2 milligrams per kilogram of diazepam was added, the respiratory rate decreased by 50%. This means benzodiazepine opioid deaths aren’t simply about adding risk—the combination multiplies it.

The research went further, examining whether benzodiazepines make it harder for air to reach the lungs. Benzodiazepines relax muscles, and the team discovered they significantly reduce airflow into the upper airways. Breathing in requires active effort, like inflating a balloon, whilst breathing out happens passively. The experiments showed that benzodiazepines particularly affect inhalation, making it even harder to get sufficient oxygen when combined with opioids’ respiratory suppression.

At very high doses, morphine alone eventually reduced the ratio of air flowing in versus out. However, when combined with even a low dose of diazepam, this effect occurred at much lower morphine doses. The combination doesn’t just slow breathing—it fundamentally impairs the body’s ability to draw air into the lungs.

The Pattern of Use

The research team interviewed 48 people across Glasgow, Teesside and Bristol who use both benzodiazepines and opioids. The interviews revealed six distinct patterns of co-use, ranging from heavy, uncontrolled binge use throughout the day to more controlled, situation-specific use.

These patterns weren’t static. A person might start the day using both substances just to function normally, but by lunchtime their motivation could shift entirely—perhaps triggered by an upsetting memory, an unexpected windfall, or a stressful encounter. This volatility makes polydrug overdose risks particularly unpredictable.

The vast majority of those interviewed reported experiencing at least one overdose, including lengthy periods of unconsciousness. Almost all described struggling with poor mental health, whether formally diagnosed conditions like post-traumatic stress disorder and psychosis, or self-reported anxiety and depression often linked to childhood trauma, abuse and neglect.

Why People Mix These Substances

The research identified two main motivations for combining benzodiazepines and opioids: functional and experiential. Functional use meant taking both substances simply to get through the day—to get out of bed, do shopping, manage basic tasks. People described using this combination to self-medicate mental health symptoms and drown out trauma.

Experiential motivations involved seeking specific feelings. Some wanted “a wee buzz” to feel energised and invincible before engaging in criminal activities like shoplifting. Others sought complete oblivion—a state where they stopped thinking, feeling and existing altogether. This desire for oblivion was directly linked to unprocessed trauma and unbearable emotional pain.

The interviews revealed a troubling paradox. People were acutely aware of overdose risks and terrified of dying. Yet in the absence of adequate mental health support and facing a choice between street drugs or nothing at all, they felt they had no alternative. One Glasgow participant captured this dilemma: “We need both. We don’t just need the medicine. We need mental health support to work alongside each other.”

The Mental Health Crisis Behind Benzodiazepine Opioid Deaths

Perhaps the most striking finding was the near-universal experience of poor mental health amongst those interviewed. The vast majority reported significant mental health challenges, yet felt unable to access appropriate support. Many described being refused mental health services because of their drug use—told they weren’t “ready” for support or that their substance use made them ineligible.

This creates a vicious cycle. People use benzodiazepines and opioids to manage unbearable mental health symptoms stemming from trauma. The substances numb the pain temporarily but prevent healing. When they try to access mental health support, they’re turned away because of their drug use. Without that support, stopping or reducing substance use becomes nearly impossible because the underlying trauma remains unaddressed.

One person explained how trauma resurfaces when substance use decreases: “When you are coming off drugs, all your emotions are coming back to you. Years when you’ve taken drugs, you’re numbing the pain. So once you are prescribed drugs, everything becomes clearer.” They described feeling like “the trauma just slapped them in the face.”

The Service Gap

The research team interviewed 24 healthcare professionals across England and Scotland, including medical prescribers, frontline staff and mental health service providers. Their responses revealed significant gaps in support and guidance.

Staff understood intellectually that people needed a safer supply of substances. They recognised the dangers of street benzodiazepines, which may contain unknown contaminants or varying potencies. However, they felt paralysed by competing concerns and lack of evidence-based guidance.

In England particularly, there is no national framework for addressing benzodiazepine use amongst people also using opioids. This led to wildly inconsistent approaches. Some organisations maintained blanket policies against prescribing any benzodiazepines. Others would prescribe short detoxification regimens but rarely longer-term support. The postcode lottery meant access to support depended entirely on where someone lived.

Scotland’s Medication Assisted Treatment (MAT) standards, published in May 2024, provide more guidance. Healthcare professionals there reported feeling more confident having a framework for discussing risks and benefits with patients. However, significant challenges remained, particularly in rural areas and for those not receiving opioid substitution therapy.

Why the Evidence Gap Matters

Healthcare professionals expressed deep uncertainty about benzodiazepine opioid deaths and how to prevent them. They worried about contributing to fatal overdoses through prescribing, but also recognised that street drug use carried enormous risks from contamination and unknown potency.

A recent review found that co-prescribing benzodiazepines and opioids increased all-cause mortality by 75 to 90%. However, the evidence specifically on overdose deaths remained less clear. Interestingly, the review suggested co-prescribing might improve treatment retention, with people staying in treatment longer.

This evidence gap leaves prescribers without clear answers. One psychiatrist summarised the situation: “The very top priority is recognising that co-use of benzodiazepines and opioids is an issue. It’s killing people. It’s making people live lives that are really not the kind of lives that they want to live. It’s a big issue and currently there’s not a lot getting done for these people.”

A new clinical trial called INBOAT, launched in November 2024 and led by Professor Katrina Mooney at the University of Stirling, will test whether tailored diazepam maintenance is more effective than standard tapering doses for reducing street benzodiazepine use. The trial includes psychosocial support addressing trauma and anxiety. Results from this trial may finally provide the evidence base that healthcare professionals desperately need.

The Complexity of Polydrug Overdose

The research highlighted that benzodiazepine and opioid use doesn’t happen in isolation. Nearly everyone interviewed reported using multiple substances, including alcohol. Around a quarter reported heavy daily alcohol consumption of up to six pints per day. This polydrug overdose pattern makes risk assessment and prevention even more complex.

Healthcare professionals reported lacking training on the pharmacology of different benzodiazepines. Street supplies might contain various formulations with different onset times and durations of action. Some act quickly, others take longer to kick in. Some stay in the system for hours, others for days. This variation matters enormously for understanding overdose risk, but frontline workers often lacked this knowledge.

People described taking “a handful” of tablets, then taking more minutes later when nothing happened—not realising they might have taken a slower-acting formulation. This pattern of re-dosing before the first dose takes effect significantly increases benzodiazepine opioid deaths.

What the Research Reveals

The findings expose a system failing vulnerable people at multiple levels. Laboratory evidence shows clearly how benzodiazepines and opioids interact to suppress breathing and reduce oxygen flow. Yet this scientific understanding hasn’t translated into effective prevention strategies or support systems.

People who use both substances feel abandoned. They’re acutely aware of the risks but see no viable alternatives. People need mental health support but are denied it because of their drug use. Many recognise the extra dangers of street drugs yet cannot access safer pharmaceutical options. Even when they want to change, barriers stand in the way at every turn.

Healthcare professionals want to help but lack guidance, evidence and confidence. They fear contributing to deaths through prescribing decisions. They recognise gaps in mental health support but work within systems with limited resources and competing priorities. Different prescribers take different approaches, creating a postcode lottery of care.

The research recommendations emphasise several urgent needs: flexible prescribing tailored to individual circumstances, specialist mental health support adapted to people who use substances, holistic psychosocial support to address underlying trauma, better training for professionals on benzodiazepine pharmacology, and crucially, addressing the evidence gaps through rigorous research.

However, none of these recommendations address the fundamental question: how do we prevent people from beginning this dangerous pattern of use in the first place? With mental health challenges, childhood trauma and adverse experiences appearing so consistently in people’s stories, prevention must start much earlier—before substance use becomes the only available coping mechanism for unbearable psychological pain.

The scale of benzodiazepine opioid deaths—70% of opioid deaths in Scotland, 93% of all drug deaths in England involving multiple substances—represents not just a treatment challenge but a fundamental failure of mental health support, trauma-informed care, and early intervention. Until we address these root causes, the death toll will continue to rise.

Source: dbrecoveryresources