Weight loss injections: experiences and what your blood values are doing

Experiences with weight loss injections resemble each other closely: nausea in the first weeks, then less hunger, and the first visible kilos somewhere in month three or four. The Dutch review site KliniekErvaringen holds more than 200 reviews about losing weight with medication, averaging 8.3 out of 10.

What almost never appears in those stories is a number from a lab. Yet a great deal that is measurable happens over those same months.

In the STEP 1 trial the average weight loss was 14.9% over 68 weeks (Wilding et al., 2021). A drop of that size touches your muscle mass, your iron stores and your pancreas.

Below, the experiences sit next to the values. Not to contradict the stories, but to complete them.

What do people losing weight with injections report?

The most commonly reported experience is that hunger falls away. People describe feeling full halfway through a plate, and the urge to keep eating disappearing. Alongside that, nausea, constipation and fatigue come up often, usually in the first weeks and just after each dose increase.

The kilos people mention vary widely. Public accounts include figures such as 12 kilos in four months and 20 kilos over roughly eight and a half months.

That spread is no accident. Dose, starting weight, how much someone moves and how much protein they eat all differ. An average from a trial says something about a group, not about you.

What is happening in your blood meanwhile?

Roughly three things move, and not at the same time. Your blood sugar values respond first, often within weeks. Your pancreas and liver values can move throughout. Your storage values, such as ferritin and vitamin B12, change slowest, because a store needs months to run down.

That last row sits outside the timeline deliberately. Severe abdominal pain is not a phase to push through, and it fits a complaint that belongs with a doctor the same day.

Why do other people experiences say little about your own?

Experience stories select themselves. People for whom it went well write a review more often than people who stopped after six weeks. On top of that they almost never state which dose they used, for how long, and what their starting values were. Without those three details a story cannot be translated to your situation.

I am not saying that to dismiss them. They are genuinely useful for what they do describe: how it feels, and when.

They are simply unsuited as a yardstick. Take two people who both lose 15 kilos in six months. In one, mostly fat mass comes off. In the other, relatively more muscle goes with it. On the scale those two look identical.

In the SEMALEAN study fat mass fell by 14% at seven months and 18% at twelve months, while lean mass first dropped by 3 kilos and then stabilised. Handgrip strength actually improved over that period (Alissou et al., 2026).

Which complaints map to a value and which do not?

Not every complaint can be read from blood. Nausea from slower stomach emptying has no value of its own. Fatigue, breathlessness and hair shedding can relate to values such as ferritin and vitamin B12. Persistent vomiting or diarrhoea can affect your salt and fluid balance, and that does show up.

Low ferritin is sometimes associated with fatigue, though other factors always play a role. The same holds for vitamin B12.

An analysis of reports involving GLP-1 drugs used for weight loss examined rarer gastrointestinal problems, including pancreatitis and bowel obstruction (Sodhi et al., 2023). Rare does not mean zero, and that is exactly why a starting value helps.

The Dutch site Thuisarts describes, per complaint, when contacting your GP makes sense. That remains the route for alarm symptoms, not a blood test.

What can you have recorded yourself?

The practical answer is a starting value, and then the same values again later. Without a starting value a result during your course is a loose number. With one, it becomes a direction. Some people choose to measure around the start and again later, and your GP can think along about what makes sense in your case.

The Dutch Voedingscentrum points out that weight loss goes together with loss of muscle mass and that adequate protein plays a role, citing 1.0 to 1.2 grams per kilogram of body weight for healthy older adults.

Our GLP-1 three month check is built around that second measurement point, assessed by a BIG-registered doctor.

If you want to know which drugs exist first, read weight loss medication compared. If you are curious about the pace, that sits in how fast Mounjaro works. And the complaints themselves are covered in Ozempic side effects.

If you are already a few weeks in without a starting value, nothing is lost. Measure today, so the measurement three months from now has something to be compared against.

References

• Wilding JPH et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity. N Engl J Med. 2021. PMID 33567185
• Alissou M et al. Impact of Semaglutide on fat mass, lean mass and muscle function in patients with obesity: The SEMALEAN study. Diabetes Obes Metab. 2026. PMID 41068996
• Sodhi M et al. Risk of Gastrointestinal Adverse Events Associated With Glucagon-Like Peptide-1 Receptor Agonists for Weight Loss. JAMA. 2023. PMID 37796527
• Voedingscentrum, factsheet on older adults and nutrition (protein requirement).
• Thuisarts.nl, guidance on when to contact your GP.

Every blood test result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.