GLP-1 side effects · Mounjaro, Wegovy, Ozempic
Fatigue in the first few weeks of Mounjaro is usually the calorie deficit and your body adjusting. Fatigue that is still there at month three, four or six is usually something the drug has quietly caused: low ferritin, low B12, low vitamin D, too little protein, or a thyroid dose that no longer matches your new weight. All of it shows up on a blood test, and all of it is fixable without stopping the medication.
Venous draw at 103 UK clinics or at home. Report within 48 hours of the lab receiving your sample. No GP referral.
Last reviewed 15 September 2026
The short answer
Persistent tiredness on Mounjaro is rarely the drug itself. It is what eating half as much for months does to iron, B12, vitamin D and protein, plus a thyroid or blood sugar picture that has shifted with the weight loss. A ferritin, B12, vitamin D, full thyroid, HbA1c and kidney and liver panel finds the cause in one draw, and most causes are corrected with food, a supplement or a dose adjustment rather than by stopping treatment.
This page is educational and does not replace your prescriber. If you have chest pain, fainting, severe abdominal pain or vomiting that will not stop, contact 111 or your prescriber today.
Why it happens
Weeks one to eight. You are eating far less, your blood sugar is running lower and flatter than it used to, you are probably slightly dehydrated because you forget to drink, and each dose increase brings a few days of nausea that wrecks sleep. This tiredness tracks the dose escalation and usually eases as each dose settles. It is expected, and a blood test is unlikely to change what you do.
Month three onwards. If you are still flat, foggy, breathless on stairs, or waking unrefreshed, the question changes. By now you have eaten 40 to 60 percent less food than usual for several months. Iron, B12 and vitamin D stores were being drawn down the whole time, and tirzepatide slows stomach emptying, which reduces how well B12 is absorbed from what you do eat. If protein has been low, some of the weight you lost was muscle, which shows up as weakness rather than lightness. And if you take levothyroxine, a dose that was right at your old weight may now be too much or too little. None of that is the drug failing. It is the drug working, without the maintenance that should come with it.
What to test
| Marker | Why Mounjaro moves it | Watch for | Fix | In panel |
|---|---|---|---|---|
| Ferritin | Less red meat and fewer fortified foods for months. Menstruating women drain stores fastest. | Below 30 is deficiency; symptoms common below 50; breathlessness, hair shedding, restless legs | Iron, usually alternate-day, retest in 12 weeks | Advanced, Ultimate, Signature |
| Vitamin B12 | Slower stomach emptying and less stomach acid reduce absorption; low intake on top | Below 300 is a grey zone; pins and needles, brain fog, low mood | High-dose oral or sublingual methyl B12; injections if very low | Advanced, Ultimate, Signature |
| Vitamin D | Rapid fat loss releases stored D initially, then intake drops and levels fall | Below 50 nmol/L is insufficient; muscle aches, low mood, poor immunity | D3 with K2, retest in 3 months | Advanced, Ultimate, Signature |
| Thyroid (TSH, free T4, free T3) | Significant weight loss changes levothyroxine requirements; undiagnosed hypothyroidism also mimics GLP-1 fatigue | TSH drifting above 2.5 with symptoms; free T3 at the bottom of range | Dose review with your prescriber | Advanced, Ultimate, Signature |
| HbA1c and fasting insulin | Blood sugar falls fast on tirzepatide; if you also take a sulfonylurea or insulin, hypos cause exhaustion | HbA1c falling steeply; symptoms after meals or overnight | Medication review; the insulin result shows how much the drug has already done | HbA1c in all; fasting insulin and HOMA-IR in Ultimate and Signature |
| Kidney, liver, electrolytes, albumin | Dehydration and low protein intake; rapid weight loss stresses the liver briefly | Low albumin (protein), raised creatinine or urea (dehydration), low sodium or potassium | Fluids, 1.2–1.6 g protein per kg, recheck | Advanced, Ultimate, Signature |
Reference points are UK laboratory conventions and NICE thresholds where they exist; your report reads each result against the others rather than in isolation. Ferritin guide.B12 guide.Vitamin D guide.Thyroid guide.HbA1c guide.
Read it in context
A GP asked about tiredness on Mounjaro will often check a full blood count and thyroid. Both can come back normal while ferritin sits at 18, because haemoglobin only falls once iron stores are exhausted, and TSH stays inside the range while free T3 has slid to the floor. The pattern is what matters: low ferritin with low B12 and low albumin says intake; low ferritin alone in a woman with heavy periods says blood loss; a normal ferritin with a raised hs-CRP says inflammation is inflating the number and the real iron picture is worse than it looks.
Pattern 1
Ferritin, B12, vitamin D and albumin all at the bottom of range. This is intake. The fix is a structured eating plan around protein and iron-rich foods, with supplements to refill stores, not a change of drug.
Pattern 2
TSH has moved since you started, or free T3 is low with normal TSH. If you take levothyroxine your dose needs reviewing at the new weight. If you do not, this may be the first sign of an underactive thyroid that was hiding behind the weight.
Pattern 3
Albumin low, creatinine falling month on month, strength dropping faster than the scale. You are losing lean mass. Protein to 1.2 to 1.6 g per kg and resistance training two or three times a week reverses it, and your later results will show it.
Which panel
Advanced
74 biomarkers · £269
Ferritin with full iron studies, B12, folate, vitamin D, full thyroid with antibodies, HbA1c, kidney, liver, electrolytes, albumin and hs-CRP. Covers all six fatigue markers except fasting insulin.
View AdvancedRight for GLP-1 users
Ultimate
114 biomarkers · £349
Everything in Advanced plus fasting insulin and HOMA-IR, which show how far the drug has moved your metabolism, complete hormones including testosterone and oestradiol, cystatin C and Lp(a). The panel to repeat every three to six months while you are on treatment.
View UltimateSignature
230 biomarkers · £799
Processed by Randox. Adds homocysteine, extended apolipoproteins, a full autoimmune and digestive profile and physical measurements. For the deepest baseline before or during a long course.
View SignatureClinic draw £19. At-home £39. Fast from 10pm the night before. Pay monthly available. Health plans bring the price down if you plan to retest.
Honest comparison
If you are a menstruating woman, four months into Mounjaro and breathless on stairs, a standalone ferritin test from Medichecks for around £39 will very likely find your answer, and it is the honest recommendation. The case for a panel is that GLP-1 fatigue usually has more than one cause at once, and the causes interact: low iron looks worse with low B12, a thyroid shift explains why the iron is not helping, and the insulin result tells you whether to push on. One draw that reads them together, with a report that says what to do in what order, is what stops the six-month guessing game. How TrueVitals compares to Medichecks.
After your result
Each low or drifting marker is explained in plain English with the others it relates to, so you know whether this is intake, absorption, thyroid, blood sugar or muscle, and which to fix first.
A thyroid dose change, a diabetes medication review or a B12 injection course all need your prescriber. The report tells you which results to show them and why, so the conversation is short. What to ask your GP for.
Iron and vitamin D take about three months to refill. Your second report compares every marker against the first, so you can see the supplements working and the thyroid settling rather than guessing from how you feel. Blood tests before and during Mounjaro.
FAQs
Yes, especially in the first weeks and for a few days after each dose increase, when reduced calorie intake, lower blood sugar and nausea all combine. Fatigue that persists beyond two to three months is not something to accept as normal; it usually has a nutritional or thyroid cause that a blood test can identify.
Ferritin with iron studies, vitamin B12, vitamin D, a full thyroid panel (TSH, free T4, free T3), HbA1c, and kidney and liver function including albumin and electrolytes. Fasting insulin is worth adding to see how far your metabolism has moved. All of these are in the TrueVitals Advanced and Ultimate panels.
Not directly, but it slows stomach emptying and you eat far less, and B12 absorption depends on both stomach acid and intake. Over months that combination commonly pushes B12 into the low or borderline range, particularly if you were already low, vegetarian, or take metformin or an acid-suppressing medicine.
Not without speaking to your prescriber. Most causes of GLP-1 fatigue, including low iron, B12, vitamin D and protein, are corrected while continuing treatment. Stopping loses the metabolic progress without fixing the deficiency that is actually causing the tiredness.
Yes: nothing but water from 10pm the night before, with a morning appointment, so that HbA1c, fasting insulin and lipids are accurate. Take your Mounjaro on its normal day; it does not need to be paused. Full preparation guide.
Yes. Semaglutide and tirzepatide reduce intake and slow stomach emptying by the same mechanism, so the same markers apply and the same tests answer the question.
Most guidance for people losing weight on a GLP-1 is 1.2 to 1.6 grams of protein per kilogram of body weight per day, prioritised at each meal, alongside resistance training to protect muscle. Your albumin and creatinine results show whether you are managing it.
One venous draw, 114 biomarkers, a report within 48 hours that says which of the six causes it is and what to do about it. Clinic or at home, no GP referral.
Mounjaro blood tests · Before you start · Always tired, tests normal · Can't lose weight · Find a clinic
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