Blood tests before and during weight-loss injections such as Mounjaro and Wegovy
Short answer: weight-loss injections such as tirzepatide (Mounjaro) and semaglutide (Wegovy) are prescription medicines. Blood tests before you start help a prescriber check that treatment is suitable, and they give you a baseline to compare against later. They support a proper prescribing consultation; they do not replace one.
Why blood tests matter with these medicines
These medicines reduce appetite and can quickly change how much you eat and drink. Blood tests can pick up conditions that affect whether treatment is suitable or how closely it should be monitored, such as raised blood sugar, kidney or liver problems and very high triglycerides, a type of blood fat linked with pancreatitis.
Tests commonly checked before starting
HbA1c: blood sugar control, which matters especially if you take diabetes medicines. See HbA1c.
Kidney function: sickness and diarrhoea can cause dehydration, which strains the kidneys. See the kidney health profile.
Liver function and cholesterol, including triglycerides: see the liver function test and cholesterol profile.
Thyroid function and a full blood count: an underactive thyroid can contribute to weight gain, and the blood count checks for anaemia.
Our weight loss commencement test is designed as a baseline before treatment starts.
Tests during treatment
Eating much less can lower your intake of iron, B12, folate and vitamin D. Repeating HbA1c and cholesterol shows how your body is responding. Your prescriber will advise how often to retest; some services do it every three to six months.
Warning signs that need urgent help
Severe, persistent pain in the upper abdomen, sometimes spreading to the back, with or without vomiting
Pain under the right ribs, yellowing of the skin or eyes, or a high temperature
Vomiting or diarrhoea that stops you keeping fluids down
Signs of low blood sugar, such as shaking, sweating or confusion, if you also take insulin or certain diabetes tablets
Contraception and pregnancy
These medicines should not be used in pregnancy. The MHRA has advised that Mounjaro may make the contraceptive pill less effective in people who are overweight, so ask your prescriber which contraception to use.
Buy only from a registered prescriber
Only get weight-loss injections through a registered prescriber or pharmacy. Products bought from social media or unregulated websites may be fake or unsafe.
Frequently asked questions
Do I need to fast before these blood tests?
Glucose and triglycerides are best measured after fasting. Check the preparation notes on the test page.
Does Easy Medical Centre prescribe weight-loss injections?
We provide the blood tests. Prescribing decisions are made by your registered prescriber, who can use your results.
I have already started treatment. Is it too late for a baseline?
No. Testing now still gives your prescriber useful information and a point to compare against later.
Sources
NICE technology appraisal TA1026: tirzepatide for managing overweight and obesity
Mounjaro KwikPen patient information leaflet (electronic medicines compendium)
MHRA drug safety advice on GLP-1 and dual GIP/GLP-1 medicines and contraception
Related reading
Clinically reviewed by Zak Toon, ACP, on 15 October 2026.
This article is general information based on UK guidance. It is not personal medical advice. If you are worried about your health or have symptoms, speak to your GP or call 111.
