Ozempic, Mounjaro and GLP-1s: Which Lab Tests to Monitor

GLP-1 drugs are now prescribed both for type 2 diabetes and for weight loss: semaglutide (Ozempic and Wegovy injections, Rybelsus — in the US now also labeled Ozempic tablets — and oral Wegovy) and tirzepatide (Mounjaro and Zepbound). In 2026 generic semaglutide also reached pharmacies in several countries. Whichever you take, the same question comes up: which lab tests to take before the first dose, and how often to repeat them.

The honest answer is short: a focused baseline panel, a few rechecks and a clear list of warning symptoms cover most people, while popular extras such as calcitonin and lipase tend to add worry rather than safety.

This continues our series on lab monitoring on medication, after which tests to monitor on metformin and what to monitor while taking statins. Treatment decisions belong to your doctor; here we cover only the lab side.

Why lab tests matter on GLP-1 drugs — and why there is no single schedule

GLP-1 (glucagon-like peptide-1) is a gut hormone involved in blood sugar control and appetite; tirzepatide also mimics a second one, GIP. The drugs act for days and slow stomach emptying, hence their common side effects: nausea, vomiting and diarrhea.

The US prescribing information for Ozempic and its counterparts for Wegovy, Mounjaro and Zepbound set no routine test schedule. They tie tests to situations: diabetes, dehydration, diabetic retinopathy and a few interacting medicines. For obesity treatment, the American Diabetes Association (ADA) adds reviews with your clinician — at least monthly for 3 months, then at least every 3 months — which need not include blood tests. So lab tests here answer three questions: is the treatment working, is it safe for you right now, and are you getting enough nutrients?

Before the first dose: a sensible baseline

Without a starting point, nobody can later tell what the drug changed. In our view, a sensible minimum for most adults — a reference point, not a label requirement — is:

• HbA1c and fasting glucose. HbA1c (glycated hemoglobin) reflects average blood sugar over the previous 2–3 months: the main treatment marker with diabetes, and a prediabetes check without it.
• A lipid panel with triglycerides, which usually fall on these drugs, while LDL cholesterol usually does not.
• ALT and AST (± GGT), a starting point for the liver.
• Creatinine with eGFR (estimated glomerular filtration rate, how well the kidneys filter), because vomiting and diarrhea can dehydrate you and, as the labels warn, injure the kidneys.

Only with a reason: ferritin (iron stores), vitamin B12 and vitamin D after a previous deficiency, bariatric surgery, celiac disease or a very-low-calorie diet — the triggers named in a 2025 joint nutrition advisory from four US obesity and nutrition societies, not a universal panel; TSH with levothyroxine or thyroid symptoms; a pregnancy test if you could be pregnant.

The history questions that matter more than any test

Before the first dose, tell your doctor if you have had:

• medullary thyroid carcinoma (MTC), a rare thyroid cancer, yourself or in your family, or multiple endocrine neoplasia type 2 (MEN 2), an inherited condition linked to it. In the US, a boxed warning rules these drugs out in that case; it is based on rodent studies, and whether they cause these tumors in humans is unknown;
• pancreatitis (inflammation of the pancreas);
• gallbladder disease or gallstones;
• diabetic retinopathy (damage to the retina caused by diabetes).

No blood test answers these questions, calcitonin included.

During treatment: what to recheck and how often

HbA1c with diabetes. The ADA Standards of Care in Diabetes 2026 advise testing at least twice a year, and more often — for example every 3 months — if you are not at your goal or your treatment has recently changed. As GLP-1 doses are raised step by step, that usually means every 3 months at first.

HbA1c and lipids without diabetes. There is no official schedule; in our view, recheck at 3–6 months if they were abnormal at baseline, otherwise once a year.

Blood glucose with insulin or a sulfonylurea (older tablets, such as glimepiride, that make the pancreas release insulin). The combination raises the risk of low blood sugar, which the labels say may be lowered by reducing the sulfonylurea or insulin dose; the Wegovy and Zepbound labels ask for glucose monitoring before and during treatment in diabetes.

Creatinine when you are losing fluids. The labels ask for kidney monitoring in people whose side effects “could lead to volume depletion, especially during dosage initiation and escalation” — a test when vomiting or diarrhea does not settle, not a monthly check for everyone.

An eye check with diabetic retinopathy. In a 2-year trial in type 2 diabetes and high cardiovascular risk, retinopathy complications occurred in 3.0% of patients on Ozempic versus 1.8% on placebo, with a larger increase in those who already had retinopathy. A later analysis linked the extra risk to a large, fast HbA1c fall in the first 16 weeks, existing retinopathy and insulin use. Hence the ADA’s 2026 chapter on retinopathy advises assessing the retina when treatment is intensified with GLP-1 drugs; tirzepatide labels carry the same class warning.

What your results will probably show

Blood sugar. In the single-drug trials in the US labels, HbA1c fell by 1.4–1.6 points on Ozempic and 1.7–1.8 on Mounjaro, from about 8%. Without diabetes the fall is small: 0.4 points versus 0.2 on placebo in STEP 1, the main Wegovy weight-loss trial. Yet in prediabetes it matters: in that trial, 84.1% of participants with prediabetes reached normal glucose levels by week 68, versus 47.8% on placebo (see also whether prediabetes is reversible).

Lipids. Triglycerides fall clearly: by 21.9% on Wegovy versus 7.3% on placebo in the main trial of the Wegovy prescribing information, and by 16.5–24.9% more than placebo on Zepbound. LDL cholesterol barely moves: down 2.5% on Wegovy versus up 1.3% on placebo, and 2.9–5.5% below placebo on Zepbound. A GLP-1 drug does not replace a statin or other treatment for LDL cholesterol if your doctor recommends one.

Liver. The Wegovy label, which added an indication for MASH (metabolic dysfunction-associated steatohepatitis, fatty liver disease with inflammation) in 2025, reports a trend toward greater falls in ALT and AST than on placebo. A falling ALT is usually good news.

What you usually do not need: calcitonin, lipase and big panels

Calcitonin, a hormone made by thyroid C-cells that can be very high in MTC, seems an obvious test, but the labels say otherwise: “Routine monitoring of serum calcitonin or using thyroid ultrasound is of uncertain value for early detection of MTC” and “may increase the risk of unnecessary procedures”. The EU label for Mounjaro even lists raised calcitonin among its common side effects, so an unneeded test can come back high and trigger further tests. If yours already has, do not ignore it: the labels call for further evaluation, which your doctor will arrange (more on calcitonin).

Lipase and amylase without symptoms. These pancreatic enzymes often rise: lipase by 39% on average on Wegovy in the weight-loss trials (not seen on placebo) and by 28–35% on Zepbound versus 5.8% on placebo. The Wegovy label calls the significance of such rises “unknown in the absence of other signs and symptoms of pancreatitis”; European labels add that raised enzymes alone do not predict it, and an analysis of large trials of liraglutide, an older GLP-1 drug, found no basis for monitoring them unless pancreatitis is suspected.

A gallbladder ultrasound “just in case”. Gallbladder problems are more common on these drugs: a 2022 meta-analysis of randomized trials in JAMA Internal Medicine found a 37% higher relative risk, about 27 extra cases per 10,000 people a year, “especially when used at higher doses, for longer durations, and for weight loss”. Still, the labels call for gallbladder studies only when gallstones are suspected.

Large “hormone” or “wellness” panels. Some online services bundle them with a GLP-1 prescription without any guideline basis, and every extra test is one more chance of a harmless out-of-range flag that costs time, money and worry. If such a report has already arrived, our guide should I worry about my blood test results shows what to check first.

Fast weight loss: nutrients and protein

Eating much less for months can leave gaps. The ADA’s 2026 chapter on obesity and weight management advises screening for micronutrient deficiencies after fast (over 4% of body weight a month) or large (over 20%) weight loss, with no universal rule on how often; it names iron, calcium, magnesium, zinc and vitamins A, D, E, K, B1, B12 and C. A large US records study it cites found a nutritional deficiency diagnosed in nearly 13% of adults within 6 months of a GLP-1 prescription and over 22% within a year, most often vitamin D — not proof that the drug caused each one, but a reason to ask.

In practice, the usual tests are ferritin, vitamin B12 and vitamin D — B12 especially if you also take metformin, which reduces its absorption over time. The joint advisory recommends re-checking nutrient levels during treatment (no fixed interval) and discusses tailored vitamin D, calcium and B12 supplements. To protect muscle, it suggests 1.2–1.6 g of protein per kilogram of body weight a day, or simply 80–120 g — something a food diary tracks better than a blood test.

Other medicines that change what to monitor

Because GLP-1 drugs slow the stomach, they can change how some tablets are absorbed:

• Levothyroxine with semaglutide tablets. In a study with oral semaglutide, exposure to levothyroxine rose by about 33%, and the labels suggest considering increased clinical or laboratory monitoring — in practice, a TSH check (timing in our levothyroxine guide).
• Warfarin with tirzepatide. The US labels for Mounjaro and Zepbound ask for monitoring of medicines with a narrow therapeutic index, where the gap between too little and too much is small, such as warfarin (see the Zepbound prescribing information): in practice, INR checks, the clotting test used to adjust warfarin.
• Insulin or a sulfonylurea. More home glucose checks, as described above.
• The pill with tirzepatide. The US labels and the MHRA guidance for patients on GLP-1 medicines advise a non-oral method or an added barrier method, such as condoms, for 4 weeks after starting and after each dose increase. The EU label for Mounjaro, by contrast, considers the interaction not clinically relevant, and for semaglutide the EU Wegovy label expects no loss of effect. Ask your doctor or pharmacist which advice applies to you.
• Planning a pregnancy. The semaglutide labels advise stopping at least 2 months before a planned pregnancy; for tirzepatide, the EU label and the MHRA say at least 1 month, and the US Zepbound label says to stop once a pregnancy is recognized. Plan the timing with your doctor.

Generic, compounded and unapproved versions

Approved generics. In 2026 semaglutide lost patent protection in several countries — according to the health-data company IQVIA, among them Canada, Brazil, China, India, Mexico and Turkey — while in the US, the UK and most of Europe it runs to 2031 or later. Canada approved its first generic in April 2026 as pharmaceutically equivalent to the brand. Same drug, same monitoring.

Compounded versions, mainly in the US, are mixed by pharmacies rather than approved. The FDA’s page on unapproved GLP-1 drugs notes that the agency “does not review compounded drugs for safety, effectiveness or quality” and, as of its latest update (31 May 2026), counts 990 adverse-event reports associated with compounded semaglutide. The ADA’s 2026 Standards of Care do not recommend them.

Unregulated sellers — beauty salons, social media, websites that skip the consultation — are what the MHRA warns against, citing counterfeit pens. If you have used one, tell your doctor.

Red flags: when to see a doctor promptly

Seek medical help promptly, or call emergency services if symptoms are severe, for:

• severe abdominal pain that does not go away, sometimes spreading to the back, with or without vomiting (possible pancreatitis);
• pain in the upper abdomen, fever, yellowing of the skin or eyes, or pale stools (gallbladder problems);
• vomiting or diarrhea that will not stop, very little urine, dizziness (dehydration, a risk to the kidneys);
• changes in vision if you have diabetes, and on semaglutide any sudden loss of vision or rapidly worsening eyesight, which the MHRA asks you to report urgently (very rare reports link semaglutide to a serious eye condition);
• sweating, shaking or confusion if you also take insulin or a sulfonylurea (possible low blood sugar);
• a lump or swelling in the neck, trouble swallowing, persistent hoarseness or shortness of breath (named in the US thyroid warning).

Tell the doctor what you take; decisions about the drug itself are theirs.

The monitoring calendar

A reference point for your doctor’s visit, not a self-prescription:

Frequently asked questions

Do I need blood tests before starting Ozempic or Wegovy? The labels do not require a standard panel, but a baseline is sensible: HbA1c or fasting glucose, a lipid panel, ALT and AST, and creatinine with eGFR. Ferritin, B12, vitamin D, TSH or a pregnancy test are added only with a specific reason, and your personal and family history matters as much as any test.

Do I need a calcitonin test? Usually not. The US labels say routine calcitonin monitoring is of uncertain value for finding medullary thyroid cancer early and may lead to unnecessary procedures; the real safety check is your personal and family history. If calcitonin was already measured and came back high, do not ignore it: your doctor will decide on further evaluation.

My lipase is high on semaglutide — should I worry? On its own, usually not. Lipase often rises on these drugs, by 39% on average on Wegovy in the weight-loss trials, and the labels call such rises of unknown significance without other signs of pancreatitis. What matters is symptoms: severe abdominal pain that does not go away, sometimes spreading to the back, needs prompt medical attention. Otherwise, mention the result at your next visit.

How often should I check HbA1c on a GLP-1 drug? With diabetes, the ADA advises at least twice a year, and about every 3 months while treatment is changing or you are above goal — usually every 3 months at first. Without diabetes there is no official schedule; rechecking at 3–6 months if the baseline was raised, then yearly, is reasonable. Your doctor sets the exact plan.

Do generic semaglutide users need different tests? No. An approved generic is the same medicine and is monitored like the brand. Compounded or unapproved products are a different matter: the tests stay the same, but your doctor should know exactly what you have been using, because the FDA does not review compounded drugs for safety, effectiveness or quality.

Conclusion

Lab monitoring on GLP-1 drugs comes down to a clear before-and-after: a baseline panel, HbA1c on schedule if you have diabetes, a yearly look at lipids, liver and kidneys, and targeted tests when symptoms or other medicines call for them. Calcitonin, lipase and big panels without a reason usually add worry, not safety.

If you want to see your baseline and follow-up reports side by side, that is what we build Wizey for: one analysis can include up to 15 files — PDFs, photos or screenshots — so this year’s and earlier results are read together, with the links between markers explained and questions prepared for your doctor. Our example report reads exactly this kind of baseline metabolic panel as one picture. It is not a substitute for a consultation and does not start or stop treatment, but a way to arrive at the appointment prepared.