King's College London

The study, published in the British Journal of Clinical Pharmacology, found that bariatric surgery patients face unique risk factors relating to overdose. This is because the surgical changes to their stomach and intestine to reduce food and nutrient absorption may also affect how they absorb drugs.

By examining data from the National Programme of Substance Use Mortality (NPSUM), the researchers conducted the first toxicology-based investigation of overdose deaths in post-bariatric surgery patients.

Multiple medications - including opioids, antidepressants and common painkillers - were detected at post-mortem in every case, and often included medications that the deceased were not actively prescribed. Opioids were found to have contributed to the deaths of nearly every person, while antidepressants were implicated in the deaths of just under half. Unlike many cases on the NPSUM database, illicit drug and alcohol use was rare. Together, this highlights how chronic pain and mental health conditions leave this group particularly vulnerable to prescription-medication harm.

Bariatric surgery is a group of surgical operations that help people lose weight by changing the stomach and sometimes the small intestine. Some surgeries, such as a gastric sleeve, reduce the size of the stomach by 70 - 80% so it can hold much less food. Others, including gastric bypass, reroute or bypass parts of the small intestine, allowing less food to be absorbed.

While bariatric surgery can be a powerful tool in reducing excess weight, previous studies have highlighted how these significant surgical changes can unpredictably alter the pharmacokinetics of gastrointestinal system, influencing how drugs are absorbed. While this differs from person to person - depending on the type of surgery, the drug involved, and other factors - these pharmacokinetic changes can increase the potency of drugs, or alternatively, decrease the potency, leading to repeated dosing.

The researchers identified 18 people with a history of bariatric surgery whose deaths were noted in the NPSUM records between 1997 and 2025. Coroners voluntarily report a death to NPSUM if psychoactive drugs were detected or implicated in causing the death, or if the deceased had a history of drug use.

Opioids were detected and contributed to the deaths of 94.4% (17 out of 18) of cases, compared to an average of just 67% across all 58,000 cases on the NPSUM database. Non-opioid analgesics, which include drugs such as paracetamol and ibuprofen, were found in 72.2% (13/18) of cases. The prevalence of these medications suggest that this patient group faces chronic pain and has risks around medication to manage this.

Depression and anxiety were common in this group, and antidepressants were identified in 83.3% (15/18) of cases and found to have contributed in just under half of deaths.

The combination of mental health diagnoses and chronic pain suggest polypharmacy – when an individual is on multiple medicines at once - could be a problem, leading to higher risk of side effects and drug interactions. The average number of drugs detected at post-mortem was six.

Coroners had determined that 75% of individuals had died of accidental overdose. This closely aligns with the NPSUM database, where 76% of deaths are classified as accidental. However, while illicit drugs and alcohol frequently contribute to deaths in the wider NPSUM dataset, such involvement was rare among the post-bariatric surgery cases.

The researchers call for more work to be done to better understand how surgery changes the pharmacokinetics of the gastrointestinal system and therefore the way drugs are absorbed. They also suggest that a clinical pharmacist is involved in discussions before surgery takes place, and that drug monitoring continues for patients following their surgery.