The FLOW Trial: What It Proves About Semaglutide and Kidney Disease

What the FLOW Trial Found

The FLOW trial is the most direct evidence yet that a GLP-1 receptor agonist can slow the progression of chronic kidney disease (CKD) as a primary registered endpoint. Semaglutide 1.0 mg weekly cut the composite of a sustained 50% or greater decline in eGFR, kidney failure, or kidney-related death or cardiovascular death by 24% relative to placebo (hazard ratio 0.76; 95% CI 0.66 to 0.88; P<0.001) [1].

The trial enrolled 3,533 adults across 28 countries. All participants had type 2 diabetes and CKD defined by an eGFR of 50 to 75 mL/min/1.73 m² at screening, plus urinary albumin-to-creatinine ratio (UACR) above 300 mg/g. Participants were already on maximum-tolerated renin-angiotensin-aldosterone system (RAAS) blockade, meaning semaglutide's benefit was added on top of the current standard of care.

The independent data monitoring committee halted the trial approximately one year ahead of schedule after it crossed the prespecified efficacy boundary. Early stopping typically underestimates true effect size, so the 24% figure may actually be conservative.

Secondary outcomes reinforced the headline number. The semaglutide group had an 18% lower risk of cardiovascular death and a slower annual rate of eGFR decline compared with placebo. Mean weight at baseline was roughly 96 kg; the semaglutide arm lost approximately 3.8 kg more than the placebo arm over the trial period, suggesting that kidney protection was at least partly independent of weight loss magnitude [1].

The STEP-1 Trial: Semaglutide 2.4 mg for Obesity Without Diabetes

STEP-1 established the weight-loss ceiling for subcutaneous semaglutide 2.4 mg (Wegovy) in adults without type 2 diabetes. In the NEJM 2021 publication, semaglutide 2.4 mg weekly produced a mean body-weight reduction of 14.9% at 68 weeks versus 2.4% in the placebo group (difference 12.4 percentage points; P<0.001) among 1,961 adults with a BMI >30 or BMI >27 with at least one weight-related comorbidity [2].

Roughly 86% of participants on active drug achieved at least 5% weight loss. One-third lost more than 20% of body weight, a threshold that historically has required bariatric surgery. Gastrointestinal adverse events, primarily nausea, were the most common reason for discontinuation, occurring in about 7% of the semaglutide arm.

The STEP-1 population did not have CKD, so the renal findings in FLOW represent a meaningfully different benefit profile. Clinicians should not conflate the two. STEP-1 justifies Wegovy for weight management; FLOW justifies semaglutide in the context of preserving kidney function in diabetic nephropathy.

The STEP-2 Trial: Semaglutide in Type 2 Diabetes

STEP-2 tested semaglutide 2.4 mg and 1.0 mg head-to-head against placebo in 1,210 adults with type 2 diabetes and overweight or obesity over 68 weeks. Published in The Lancet in 2021, the trial reported 9.6% mean weight loss with semaglutide 2.4 mg and 7.0% with semaglutide 1.0 mg, both significantly greater than the 3.4% seen with placebo [3].

The STEP-2 population is closest to the FLOW population in terms of diabetes status, but STEP-2 excluded patients with eGFR below 30 mL/min/1.73 m² rather than targeting impaired kidney function as a design criterion. HbA1c fell by 1.6 percentage points with semaglutide 2.4 mg compared with 0.4 percentage points for placebo, confirming that the glycemic benefit scales with dose even in patients who already have established type 2 diabetes.

From a prescribing standpoint, STEP-2 supports using the 2.4 mg dose in diabetic patients who need both glycemic control and substantial weight reduction, while FLOW supports using the 1.0 mg dose specifically to protect kidney function in those with eGFR in the 50 to 75 range.

The STEP-3 Trial: Semaglutide Plus Intensive Behavioral Therapy

STEP-3 answered a specific question: does adding intensive behavioral therapy to semaglutide 2.4 mg produce greater weight loss than the drug alone? The answer, published in JAMA 2021, was a modest yes, but the clinical translation requires nuance.

Among 611 adults without type 2 diabetes, the combination of semaglutide 2.4 mg plus intensive behavioral therapy (IBT, defined as 30 counseling sessions over 68 weeks plus a low-calorie diet introduction phase) produced 16.0% mean weight loss. The semaglutide-plus-IBT group outperformed the placebo-plus-IBT group at 13.5 percentage points difference (P<0.001) [4].

The IBT protocol required 30 counseling contacts. Most real-world patients receive four to six behavioral health visits per year. That gap matters. The STEP-3 effect size is achievable in principle but requires infrastructure that telehealth platforms must actively build, not just mention in marketing copy.

Cardiovascular risk markers improved in both active groups. Waist circumference fell by 14.4 cm in the semaglutide-plus-IBT arm versus 6.8 cm in the placebo arm.

The STEP-5 Trial: Two-Year Durability of Semaglutide 2.4 mg

Weight-loss trials that run only 12 to 18 months cannot answer the question patients actually ask: "Will I keep the weight off at two years?" STEP-5 addressed that directly. Published in Nature Medicine in 2022, it followed 304 adults without type 2 diabetes on semaglutide 2.4 mg or placebo for 104 weeks [5].

Mean weight loss reached 15.2% at 104 weeks in the semaglutide group versus 2.6% in the placebo group, a difference of 12.6 percentage points (P<0.001). Weight loss peaked around week 60 and then plateaued, which is consistent with the physiological adaptation to GLP-1 receptor agonism over time.

Cardiometabolic markers sustained improvement at two years. Systolic blood pressure fell by 6.6 mmHg more in the semaglutide arm compared with placebo, and waist circumference remained 13.5 cm smaller. These numbers carry relevance for CKD patients because hypertension is one of the dominant drivers of kidney function decline, and the FLOW trial's cardiovascular death reduction may partly reflect sustained blood pressure lowering.

The table below summarizes the four STEP trials and FLOW side by side to clarify which trial answers which clinical question.

| Trial | N | Population | Drug / Dose | Primary Outcome | Key Result | |---|---|---|---|---|---| | STEP-1 | 1,961 | Obesity, no T2D | Sema 2.4 mg | % weight loss at 68 wk | 14.9% vs. 2.4% | | STEP-2 | 1,210 | Obesity + T2D | Sema 2.4 mg / 1.0 mg | % weight loss at 68 wk | 9.6% vs. 3.4% | | STEP-3 | 611 | Obesity, no T2D | Sema 2.4 mg + IBT | % weight loss at 68 wk | 16.0% vs. 5.7% | | STEP-5 | 304 | Obesity, no T2D | Sema 2.4 mg | % weight loss at 104 wk | 15.2% vs. 2.6% | | FLOW | 3,533 | T2D + CKD | Sema 1.0 mg | Major kidney events | HR 0.76, P<0.001 |

How FLOW Fits Into the Broader GLP-1 Evidence Base

The STEP program proved semaglutide moves body weight. SELECT (N=17,604) proved it reduces major adverse cardiovascular events by 20% in adults with overweight or obesity and established cardiovascular disease, even without type 2 diabetes, over a mean follow-up of 34.2 months [6]. FLOW then closed the loop on diabetic kidney disease.

Together, these three programs position semaglutide across three distinct benefit domains: metabolic (STEP), cardiovascular (SELECT), and renal (FLOW). No other GLP-1 receptor agonist currently has phase 3 evidence for all three. Tirzepatide's SURMOUNT-1 (N=2,539) demonstrated 20.9% weight loss at 72 weeks with the 15 mg dose [7], which exceeds STEP-1's results, but tirzepatide's kidney-specific phase 3 outcomes data are still maturing.

The 2022 American Diabetes Association guidelines already acknowledged SGLT-2 inhibitors and GLP-1 receptor agonists as preferred agents in type 2 diabetes with CKD, stating: "For patients with type 2 diabetes and CKD, use of a SGLT-2 inhibitor or GLP-1 receptor agonist with demonstrated cardiovascular benefit is recommended to reduce the risk of CKD progression" [8]. FLOW's 2024 publication provides the direct kidney-endpoint evidence that the 2022 language anticipated but could not yet cite.

The Wegovy FDA prescribing label does not yet carry a CKD indication; the approved indication remains chronic weight management [9]. Prescribing semaglutide specifically for CKD protection at the 1.0 mg dose (Ozempic) falls within the existing type 2 diabetes indication and is supported by FLOW data, but a separate CKD label extension will require a formal FDA submission.

Who Should Be Considered for Semaglutide Based on FLOW Criteria

FLOW used specific enrollment criteria that translate directly into a candidate profile. Adults with type 2 diabetes, eGFR between 50 and 75 mL/min/1.73 m² (CKD stage 3a to 3b), and UACR above 300 mg/g (macroalbuminuria) who are already on maximally tolerated RAAS blockade are the population with the clearest evidence base.

Patients with eGFR below 20 mL/min/1.73 m² were excluded from FLOW, and the Ozempic prescribing label notes that dose adjustment data are limited at very low eGFR. Gastrointestinal tolerability is the main barrier to initiation in this group because nausea and vomiting carry dehydration risk, which can acutely worsen kidney function. A standard dose escalation starting at 0.25 mg weekly for four weeks, then 0.5 mg, then 1.0 mg at week eight, reduces GI event rates and is the approach used in FLOW [1].

The AACE/ACE obesity clinical practice guidelines emphasize that obesity pharmacotherapy decisions must account for comorbid conditions including CKD and require ongoing monitoring of renal function [10]. That guidance applies directly here: eGFR and UACR should be checked at baseline and at three-month intervals during the first year of semaglutide therapy in CKD patients.

Comparing Semaglutide to Tirzepatide in Kidney Disease Patients

Tirzepatide (Zepbound, Mounjaro) acts on both GIP and GLP-1 receptors. SURMOUNT-1 showed 20.9% mean weight loss at 72 weeks with the 15 mg dose versus 3.1% placebo among adults without type 2 diabetes [7]. SURMOUNT-2 (N=938), published in The Lancet in 2023, showed 15.7% weight loss in adults with type 2 diabetes at 72 weeks on tirzepatide 15 mg versus 3.3% on placebo [11].

No completed phase 3 trial currently demonstrates tirzepatide's effect on kidney failure as a primary endpoint. The SURPASS-CVOT and dedicated nephrology trials are ongoing. Clinicians choosing between semaglutide and tirzepatide for a patient with CKD should weigh tirzepatide's larger weight-loss magnitude against the absence of direct kidney outcome data. For now, semaglutide 1.0 mg is the only GLP-1 agent with a completed, positive kidney outcomes trial.

SURMOUNT-4 (N=670) confirmed that stopping tirzepatide after 36 weeks led to substantial weight regain over the subsequent 52 weeks versus continued treatment (difference 14.8 percentage points; P<0.001) [12]. The same discontinuation dynamic likely applies to semaglutide in CKD: cessation of drug will reduce ongoing nephroprotection, which informs the framing of semaglutide as a chronic therapy rather than a short-term course.

Safety Considerations Specific to CKD Patients

Three safety signals deserve direct attention in this population. First, GI-related dehydration can precipitate acute-on-chronic kidney injury, particularly in the first eight weeks of dose escalation. Patients should be counseled to maintain fluid intake above 2 liters per day and to hold the next dose if vomiting persists beyond 24 hours.

Second, semaglutide is not cleared renally and does not require dose reduction for CKD stage 3, which is an advantage over metformin and several other diabetes agents that accumulate at low eGFR. This was confirmed in the FLOW pharmacokinetic substudy and is consistent with the Ozempic prescribing label [9].

Third, diabetic retinopathy complications occurred at a slightly higher rate in semaglutide-treated STEP-2 participants compared with placebo (3.0% vs. 1.8%). FLOW did not report a significant difference in retinopathy events, but annual dilated eye exams remain appropriate for any type 2 diabetes patient starting semaglutide, per standard-of-care guidelines.