How does my obesity medication work, and which side effects need a call?

Most people start a new prescription with two stories already in their head. One is a friend who felt sick for a month and stopped. The other is a headline about eyesight, the thyroid or mood.

Neither is the whole picture. Both have numbers, and the numbers are calmer than the stories.

What does the medication do inside your body?

It copies a hormone your gut already makes. After a meal, the intestine releases GLP-1, a messenger that the brain, the stomach and the pancreas all listen for. Semaglutide and liraglutide mimic it; tirzepatide mimics it and a second gut hormone, GIP. A 2023 review in Diabetologia lays out where the signal lands:

• The brain's appetite centres turn down. Hunger arrives later and food noise gets softer.
• The stomach empties more slowly, so fullness lasts longer after a smaller meal.
• The pancreas releases insulin mainly when blood sugar is high.

A slower stomach is part of how the drug works, and one reason for the queasiness. The medication eases the defence we described in why your body fights weight loss. It does not replace eating, sleeping or moving. Two older medications, naltrexone with bupropion and orlistat, work differently and have different side effects; this article is about the gut-hormone family.

Why does the dose start so low?

Because the gut needs weeks to adjust. Think of the dose as a dimmer rather than a switch: it comes up one notch at a time, and it can come back down a notch. Health Canada's product monograph for semaglutide climbs from 0.25 mg to 2.4 mg over sixteen weeks, one step every four weeks. Tirzepatide starts at 2.5 mg and rises by 2.5 mg no sooner than every four weeks. Your prescriber sets the pace.

In the pooled semaglutide trials, stomach effects bunched up around each step and then eased. So a step can wait. The Canadian semaglutide label even allows dropping back to 1.7 mg for up to four weeks. A paused climb is a normal adjustment, the same way a lapse is not a relapse.

Which side effects are common?

Nausea, diarrhoea, constipation and vomiting. In the Canadian semaglutide monograph, nearly half of people reported nausea, against about one in six on placebo. For tirzepatide it is roughly one in four to three in ten, against about one in twelve. In the pooled semaglutide trials, ninety-eight in a hundred stomach events were mild to moderate. About seven in a hundred people on semaglutide stopped because of side effects, against three on placebo.

Feeling sick is not what makes the weight come off. In the pooled semaglutide trials, people with and without stomach effects lost a similar amount. General habits that many people find help, and that your prescriber may tailor:

• Smaller meals, and stopping at the first sign of fullness.
• Sipping fluids through the day.
• Eating protein first, as we covered in losing weight without losing strength.
• Staying upright for a while after eating.

Which symptoms mean call today?

Most people never meet any of these. They're listed so you'd recognise one:

• Severe belly pain that won't settle, sometimes through to the back, with or without vomiting. This can be pancreatitis. In the semaglutide weight trials it came to about two cases per thousand people a year, against fewer than one on placebo. In people with type 2 diabetes, a 2020 analysis of the large heart-outcome trials found no significant increase.
• Pain under the right ribs, fever, or yellowing skin or eyes. This can be gallstones: about three in two hundred on semaglutide, against fewer than one in a hundred on placebo. A 2022 analysis of seventy-six trials found the risk higher in weight-loss trials, at higher doses and with longer use.
• Not keeping fluids down. Dehydration can strain the kidneys.
• Sudden loss or blurring of vision in one eye, even partial. That needs an urgent eye assessment.
• New or worsening low mood, or thoughts of self-harm. In Canada you can call or text 9-8-8 at any hour.
• A lump in the neck, hoarseness or trouble swallowing.
• Swelling of the face or throat, or trouble breathing. Call 911.

And before any procedure with sedation or a general anaesthetic, tell the surgical team and your prescriber. A slow stomach may still hold food, and both Canadian weight-management monographs warn about it.

What about the headlines?

Each one is rare. None is ignored.

Eyesight. NAION is a sudden loss of blood flow to the front of the optic nerve. A 2024 study from a single eye clinic raised the alarm. A 2025 study across fourteen databases and thirty-seven million people with type 2 diabetes found a modest increase, smaller than first reported. The European Medicines Agency concluded in June 2025 that it is very rare with semaglutide, up to one person in ten thousand. In Canada, Health Canada's tirzepatide monograph has carried a warning since June 2026; the semaglutide monograph had not added one when we checked in September 2026.

The thyroid. The warning comes from rodent studies, and a 2023 French study raised a signal. A 2024 Scandinavian registry study of nearly a hundred and fifty thousand people with type 2 diabetes found no increase in thyroid cancer.

Mood. A 2024 study of almost a quarter of a million people with overweight or obesity found no increase in suicidal thoughts on semaglutide, compared with other obesity medications. The Canadian monographs still advise against these medications after a suicide attempt or with active suicidal thoughts.

How we approach this at GOALS

GOALS is the obesity and lifestyle medicine program at Guelph Internal Medicine Clinic, and the medical care is covered by OHIP, by referral from your family doctor or nurse practitioner. Your physician works out with you whether a medication belongs in your plan, which one, how fast to climb and when to hold, and you revisit it together at follow-up. A side effect is something to raise with your physician, not something to sit through alone.

Before your first dose or your next step, write down the one symptom you'd want to ask about. That note is where the next adjustment starts.