Closer monitoring of prescription drugs needed following weight loss surgery
Research highlights risk of overdose due to changes in ways body absorbs drugs after weight loss surgery.
Researchers are advising that people who have had weight loss surgery may need closer monitoring of their prescription medication to avoid risk of harm, following a recent study.
Researchers at King’s College London found that bariatric (weight loss) surgery patients faced unique risk factors relating to overdose, due to surgical changes to their stomach and intestine to reduce food and nutrient absorption that could also affect how they absorb drugs.
The findings of the study were recently published in the British Journal of Clinical Pharmacology.
Symptoms of depression, anxiety and antidepressant use were common features in the study, consistent with previous research reporting high psychiatric morbidity pre-surgery and post-surgery.
The report said: “While bariatric procedures often improve obesity-related comorbidities (e.g., type 2 diabetes and heart disease), psychological outcomes are more variable, with some patients experiencing persistent or worsening mood symptoms: a recent meta-analyses showed increased long-term suicide risk compared with the general population.”
Researchers examined figures from the National Programme of Substance Use Mortality (NPSUM), and 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 of the cases on the NPSUM database, illicit drug and alcohol use was rare.
Researchers believed their findings highlighted how chronic pain and mental health conditions leave weight loss surgery patients particularly vulnerable to prescription-medication harm.
Dr Caroline Copeland, Senior Lecturer in Pharmacology and Toxicology, King’s College London, said: “Our research highlights a gap in care that is readily addressable. Post-bariatric surgery patients are frequently seen by their clinicians in routine follow-up appointments, into which advice about drug safety could be integrated. Embedding pharmacist-led medication reviews into standard follow-up could prevent avoidable overdose deaths in this vulnerable group.”
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.
Researchers noted that bariatric surgery can be a powerful tool in reducing excess weight, though recent research has highlighted additional risks posed to the body following procedures.
Previous studies have highlighted how the significant surgical changes following weight loss surgery can unpredictably alter the pharmacokinetics of the gastrointestinal system, influencing how drugs are absorbed.
Depending on the type of surgery, the drug involved, and other factors, this can differ from person to person, however, researchers found that these pharmacokinetic changes can increase the potency of drugs, or alternatively, decrease the potency, leading to repeated dosing.
The study team 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 painkillers, which include drugs such as paracetamol and ibuprofen, were found in 72.2% (13/18) of cases. The prevalence of these medications suggested that this patient group faced chronic pain and had 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 correlated with the NPSUM database, where 76% of deaths were classified as accidental.
Researchers commented that 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 report concluded: “Bariatric surgery patients face unique overdose risks due to altered pharmacokinetics and comorbidities of chronic pain and mental health conditions. Prescribers should be cautious when initiating or adjusting opioid, antidepressant or other pH-/formulation-sensitive medications.”
Researchers called 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 suggested that a clinical pharmacist should be involved in discussions before surgery takes place, and that drug monitoring continues for patients
following their surgery.
Dr Alice Oborne, Medicine Safety Lead, Guy’s and St Thomas’ NHS Foundation Trust, said: “It is not clear the extent to which healthcare providers in the UK include routine assessment of medicines before and after bariatric surgery. However, the deaths link to prescription medicines outlined in this study suggest that there is a need for this.
Clinical pharmacists are in a good position to review medicines after bariatric surgery considering the ongoing changes in pharmacokinetics and how patients respond to
drugs, their side effects, drug-drug interactions and other adverse drug reactions.”
Read the full report in British Journal of Clinical Pharmacology
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