Do GLP-1 Drugs Raise Anemia Risk?

A review published September 17, 2026 in the journal Medical Sciences asks a question few people connect to weight loss drugs: could semaglutide, tirzepatide, and other incretin-based therapies be quietly suppressing red blood cell production? The authors, physicians at the University Hospital of Split in Croatia, reviewed the existing evidence linking GLP-1 class medications to anemia and nutrient deficiency^[1]. Their answer is not a warning to stop taking these drugs. It is a call for closer bloodwork, because the observational signal is real even though proof of a direct cause is not.

What the review actually found

Mizdrak and colleagues surveyed the incretin drug class, from the older DPP-4 inhibitor gliptins through GLP-1 receptor agonists like semaglutide to newer multi-receptor agents like tirzepatide and retatrutide. Their central hypothesis is mechanical rather than mysterious. These drugs work partly by making people eat less and feel full longer. Reduced intake, lower dietary variety, gastrointestinal side effects and delayed stomach emptying can each chip away at the nutrients red blood cells need to form. The review names iron, vitamin B12, vitamin B2, vitamin D, calcium, magnesium and zinc as the deficiencies reported most often. Thiamine, folate, vitamin A and potassium show up less frequently^[1].

The numbers behind the concern

The strongest data point comes from a retrospective analysis of insurance claims covering 461,382 adults newly prescribed a GLP-1 receptor agonist. Among patients with no prior nutritional diagnosis, 12.7% developed a new nutrient deficiency within six months and 22.4% within a year. Vitamin D deficiency alone rose from 7.5% at six months to 13.6% at twelve. Nutritional anemia and iron deficiency anemia both appeared in the dataset as downstream complications rather than headline outcomes, which is part of why they have been easy to overlook^[2].

A separate pooled analysis of six studies covering 480,825 adults on semaglutide or tirzepatide, summarized by Harvard Health in early 2026, put a rough number on the anemia question directly: about 4% of patients developed anemia tied to nutritional deficiency, 3.2% developed iron deficiency, and 2.6% developed a low B vitamin reading^[3]. Those are minority outcomes, not a majority effect, and the underlying studies measured different populations in different ways, which is exactly why a pooled estimate carries less weight than a single well-designed trial would.

Why the case for causation is still open

The review's own conclusion is careful on purpose: current evidence is insufficient to establish a direct causal relationship or a class wide adverse effect^[1]. People starting semaglutide or tirzepatide for obesity or type 2 diabetes often begin with worse baseline nutrition than average, whether from limited diet diversity or the metabolic strain of the underlying disease itself. An observational study cannot easily separate a drug effect from a preexisting deficiency that the drug simply made easier to notice, once appetite and weight both dropped and a doctor ordered bloodwork. Age, sex, dose and treatment duration all vary between the studies cited above, and none of them randomized patients to a placebo arm the way a proper safety trial would.

Part of the problem is that the trials that won these drugs approval never looked. A 2026 analysis of GLP-1 receptor agonist trials found that only 28% of the studies assessed dietary intake at all, and just 8% reported the results^[4]. None of the major approval trials for semaglutide or tirzepatide were designed to catch a slow nutritional drift, so the anemia signal is surfacing years later in claims data and case reports instead of in the data that got the drug approved.

What we still don't know

Several questions remain open. The review does not identify which specific incretin drug carries the highest risk, and newer triple-receptor agents like retatrutide, which activates GLP-1, GIP and the glucagon receptor at once, have not been in wide use long enough to generate the multi-year claims data the older drugs now have. It is also unclear whether the anemia signal reverses with supplementation, whether it tracks with how much weight a person loses, or whether it clusters in specific groups, such as menstruating women who already carry a higher baseline risk of iron deficiency. None of that gets settled until someone designs a study built to answer it, rather than noticing the pattern inside someone else's dataset. Until then, the honest summary is a hypothesis worth monitoring, not a confirmed side effect worth a boxed warning.

What this means if you are on a GLP-1 medication

This is not a reason to stop semaglutide, tirzepatide or any other incretin therapy on your own. It is a reason to ask your prescriber about baseline bloodwork before you start, and a repeat check a few months in, particularly if you notice fatigue, pale skin, shortness of breath or dizziness alongside the appetite changes the drug is supposed to cause. If you are considering any peptide in this class, the regulatory status of what you would be taking matters as much as the nutrition question. Our semaglutide and tirzepatide regulation pages break down prescription status by country.