GLP-1 Drugs: Semaglutide, Tirzepatide Show Varying Benefits, Harms in T2D

Type 2 diabetes remains a leading cause of morbidity and mortality worldwide, driven largely by its associated macrovascular and microvascular complications. Despite a growing armamentarium of glucose-lowering agents, a significant unmet need persists for therapies that not only achieve stringent glycaemic control but also provide organ protection and improve quality of life. The GLP-1 receptor agonists have emerged as a cornerstone of T2D management, but their individual profiles, particularly regarding cardiovascular outcomes, weight reduction, and specific adverse events, have warranted a head-to-head comparison to guide clinical decision-making.¹

A living systematic review and network meta-analysis, published in the BMJ, aimed to provide up-to-date evidence on the key benefits, harms, and uncertainties surrounding medications for adults with type 2 diabetes. Nong, Jeppesen, and Shi conducted an exhaustive search of MEDLINE, Embase, and Cochrane Central Register of Controlled Trials from inception through October 2024, identifying 453 randomised controlled trials involving 21 different drug classes and over 300,000 participants. The analysis included direct and indirect comparisons of monotherapies, dual therapies, and triple therapies, with a particular focus on GLP-1 receptor agonists and their newer dual-agonist counterparts.¹

Comparing the numbers on GLP-1s

The analysis confirmed that GLP-1 receptor agonists, as a class, significantly reduced HbA1c compared to placebo. Among the GLP-1 receptor agonists, tirzepatide, a dual GIP/GLP-1 receptor agonist, demonstrated the most potent glycaemic lowering, reducing HbA1c by 1.86% (95% CI, -2.06 to -1.66) compared to placebo. Semaglutide (subcutaneous) followed closely, with an HbA1c reduction of 1.48% (95% CI, -1.60 to -1.36) versus placebo. Oral semaglutide reduced HbA1c by 1.05% (95% CI, -1.19 to -0.91) compared to placebo. These reductions were generally superior to dipeptidyl peptidase-4 (DPP-4) inhibitors and sulfonylureas.¹

Cardiovascular outcomes represented a critical endpoint. The network meta-analysis found that GLP-1 receptor agonists, as a class, reduced the risk of major adverse cardiovascular events (MACE) by 16% (HR 0.84; 95% CI, 0.79-0.89) compared to placebo or standard care. Specifically, semaglutide (subcutaneous) showed a MACE reduction of 20% (HR 0.80; 95% CI, 0.73-0.88), while tirzepatide reduced MACE by 18% (HR 0.82; 95% CI, 0.74-0.91). Liraglutide also demonstrated a significant MACE reduction of 13% (HR 0.87; 95% CI, 0.81-0.94). These benefits were consistent across patients with established cardiovascular disease and those with multiple cardiovascular risk factors.¹

Weight management is another key benefit of GLP-1 receptor agonists. Tirzepatide led to the greatest weight loss, with a mean difference of -10.5 kg (95% CI, -11.9 to -9.1) compared to placebo. Subcutaneous semaglutide resulted in a mean weight loss of -6.5 kg (95% CI, -7.2 to -5.8) versus placebo, while oral semaglutide achieved -4.1 kg (95% CI, -4.8 to -3.4). These weight reductions were substantially higher than those observed with other non-GLP-1 receptor agonist classes, highlighting the unique metabolic advantages of these agents beyond glycaemic control.¹

Renal outcomes were also assessed, with GLP-1 receptor agonists demonstrating a class effect in reducing the risk of new-onset or worsening nephropathy. The analysis showed a 21% reduction in the composite renal endpoint (HR 0.79; 95% CI, 0.73-0.86) for GLP-1 receptor agonists compared to placebo. Semaglutide (subcutaneous) specifically reduced this risk by 24% (HR 0.76; 95% CI, 0.68-0.85), and tirzepatide by 20% (HR 0.80; 95% CI, 0.71-0.90). This renal protection is a critical consideration for T2D patients, who often face progressive kidney disease.¹

But the benefits come with a cost in terms of adverse events. Gastrointestinal side effects, particularly nausea, vomiting, and diarrhoea, were common across all GLP-1 receptor agonists. Tirzepatide had the highest rates of nausea (OR 3.5; 95% CI, 2.9-4.2) and vomiting (OR 3.1; 95% CI, 2.5-3.8) compared to placebo, followed by subcutaneous semaglutide. These events often led to treatment discontinuation, though most were mild to moderate and transient. Pancreatitis and gallbladder-related events were rare but consistently reported across the class, requiring careful patient selection and monitoring.¹

A separate population-based cohort study, published in Diabetes, Obesity and Metabolism, specifically investigated the risk of suicidal ideation and suicidality among adults prescribed semaglutide for weight management. Her, Wang, and Stürmer analysed electronic health records of over 1.5 million adults in the US, comparing semaglutide users to those prescribed other anti-obesity medications or no anti-obesity medication. The study found no increased risk of suicidal ideation or suicidality in patients initiating semaglutide compared to those initiating other anti-obesity medications (HR 0.98; 95% CI, 0.85-1.13) or those not receiving anti-obesity medication (HR 1.02; 95% CI, 0.90-1.15). The absolute risk of these events remained low across all groups.²

This finding is important given recent regulatory scrutiny and patient concerns regarding potential psychiatric adverse events with GLP-1 receptor agonists. The cohort study provided reassurance that, in a real-world setting, semaglutide does not appear to elevate the risk of suicidal ideation or suicidality beyond what is observed with other weight management interventions or in the general population. This large-scale observational data complements the safety profiles established in randomised controlled trials, which typically exclude patients with severe psychiatric comorbidities.²

The living systematic review had several strengths, including its comprehensive search strategy and the use of network meta-analysis, which allowed for indirect comparisons between drugs not directly tested against each other. Still, the inherent heterogeneity across trials in terms of patient populations, baseline characteristics, and concomitant medications introduces some uncertainty. The follow-up duration for some trials was also relatively short, potentially limiting the detection of rare or long-term adverse events. The analysis also relied on published data, meaning it was subject to reporting biases present in the original trials.¹

The population-based cohort study on suicidality, while large, was observational. This means it cannot definitively establish causality and may be subject to unmeasured confounding factors, despite rigorous statistical adjustments. Patients with a history of severe psychiatric illness or active suicidal ideation might have been less likely to be prescribed semaglutide, potentially biasing the results towards a lower observed risk. Future studies, perhaps with longer follow-up and more granular data on psychiatric history, could further refine these safety assessments.²

The question remains whether the cardiovascular and renal benefits observed with GLP-1 receptor agonists are solely due to glycaemic control and weight loss, or if there are direct pleiotropic effects on these organ systems. Further mechanistic studies are needed to fully elucidate these pathways. Additionally, the long-term cost-effectiveness of these agents, particularly the newer dual agonists, in diverse healthcare systems warrants ongoing evaluation.