Side effects, week by week

The most useful thing to know about GLP-1 side effects is that they have a shape. They are not a constant background level of feeling unwell, and they are not random bad days. They arrive in a wave after each dose increase and recede as your body adjusts to the new level.

Once you can see that shape, two things change. You stop interpreting a bad week as the medicine being wrong for you. And you can plan around the days that are predictably harder.

The rhythm

Recall that a new dose takes weeks to reach its full blood level. Symptoms track that climb. In practice this produces a repeating cycle:

With weekly injections, many people also notice a milder version of this within each week — a heavier day or two after the injection, easing toward the end of the cycle. If that pattern holds for you, injecting on a day where the following evening is quiet is a small change that makes a real difference.

The common effects, and what is behind each

Nausea

The most common by a wide margin. It comes from delayed gastric emptying — food is still in your stomach when your habits say it should not be. The strongest predictor of a bad day is eating the volume you used to eat. Smaller portions, eaten slowly, stopping at the first sign of fullness rather than at the end of the plate: unglamorous, and more effective than anything else.

Fatty and fried food sits longest and provokes the most. Very sweet food often follows.

Constipation

Slower gut motility plus less food plus — very often — less water. Intake falls without anyone deciding it should, and fluid intake falls with it. This is the effect most likely to persist rather than fade, and the one most responsive to boring interventions: fluid, fibre, movement.

Diarrhoea

Less common than constipation but not rare, and sometimes alternating with it. Worth mentioning if it lasts more than a few days, because dehydration compounds every other symptom.

Reflux and burping

Stomach contents sitting longer, with more pressure behind them. Sulphurous burps are a frequently reported and rarely discussed one. Eating earlier in the evening and staying upright afterwards helps more than most remedies.

Fatigue

Usually not the drug itself so much as the consequence of eating markedly less than before. If tiredness is prominent, the first thing to check is whether total intake has collapsed rather than merely reduced — and whether protein in particular has fallen away.

What helps, in order of how much it helps

1. Smaller portions. The stomach's capacity has effectively shrunk. Serving the same amount and stopping early is harder than serving less.
2. Slow down. Fullness signals arrive late. Eating fast means eating past the signal before it lands.
3. Fluid, deliberately. Thirst cues weaken along with hunger cues. Most people need to make drinking a scheduled thing rather than a responsive one.
4. Move the fat and sugar down the plate. Both delay emptying further, on top of a stomach already delayed.
5. Ask about extending a step. Staying four more weeks at a tolerable dose costs almost nothing and rescues a lot of treatments.

When it is not a side effect

The line is worth stating plainly, because the ordinary side effects are common enough that people learn to dismiss abdominal symptoms. Ordinary nausea comes in waves, eases between meals, and improves over a week. Pain that is constant, severe, and radiating is a different thing entirely.

Keeping a record that is worth keeping

Two or three cycles of notes turn a vague impression into something a clinician can act on. The useful record is short: which rung you are on, the date it started, which day symptoms peaked, and whether they had settled by the third week.

That is four pieces of information and it answers the questions a consultation actually turns on. A long symptom diary is harder to keep and, in practice, less useful than those four lines recorded consistently.

Related

• Injection sites, and why rotation matters — Three approved areas, one simple rule, and a failure mode that quietly reduces how much drug you absorb.
• Constipation, and what actually helps — The side effect most likely to persist rather than fade — and the one most responsive to three unglamorous changes.

Which medicine you are on

Everything above applies across the class, because it comes from a shared mechanism. The numbers that differ — dose ladders, missed-dose windows, in-use storage periods, half-lives — differ by product, sometimes substantially. Each medicine has its own reference here:

Ozempic · Wegovy · Mounjaro · Zepbound · Rybelsus · Saxenda · Victoza · Trulicity · all medicines