GLP-1 Side Effects: What to Expect, and What Needs a Doctor

The common ones, and when they show up

Slowed stomach emptying is not a side effect of these medicines so much as part of how they work, and almost everything below follows from it.

• Nausea. The most commonly reported effect. Usually strongest in the first day or two after an injection and in the week after a dose increase.
• Constipation. Very common and often the one that persists longest, partly because people are eating far less fibre than before.
• Diarrhoea. Less common than constipation but frequent enough to plan around early on.
• Reflux, burping and a sulphurous taste. Reported often and rarely discussed on marketing pages.
• Early fullness and food aversions. Many people find specific foods, often fatty or very sweet ones, become unappealing.
• Fatigue and light-headedness. Frequently a knock-on effect of eating and drinking much less rather than of the medicine directly.
• Injection-site reactions. Usually mild and short-lived.

These are typically most intense in the first weeks and after each step up in dose, then settle. If they do not settle, that is information your prescriber needs, not something to push through.

The ones that need medical attention

The approved labelling for these medicines also carries a boxed warning about thyroid C-cell tumours observed in rodents, and they are not used in people with a personal or family history of medullary thyroid carcinoma or MEN 2. Other documented cautions include pancreatitis, gallbladder disease, kidney injury from dehydration, and worsening of diabetic retinopathy in some people with diabetes. None of these are common. All of them are reasons the medicine needs a prescriber who knows your history.

They are also not appropriate in pregnancy, and anyone who could become pregnant should discuss contraception and planning before starting.

Why the dose schedule exists

Treatment starts low and increases in steps, typically over several months, and the starting dose is chosen for tolerance rather than for effect. Going up too quickly is the single most reliable way to make side effects unbearable; going up too slowly delays any benefit.

That balance is a clinical judgement. It depends on how you are tolerating the current dose, what else you take, and what your response has been. It is not something to self-manage from a vial and a chart, and no reputable prescriber will hand that decision to you.

If a plan makes it hard to reach a prescriber between shipments, that is a real cost. It is worth checking before you buy how you contact a clinician, how quickly they respond, and whether it costs extra.

The risks that come from buying compounded, not from the molecule

These are specific to this market and worth knowing about.

Dosing units. Compounded preparations are often supplied in multi-dose vials at concentrations chosen by the pharmacy, and instructions may be given in units on a syringe rather than in milligrams. That conversion is where documented dosing errors have happened. If the number on your instructions and the number on the vial are in different units, stop and confirm before you inject anything.

Cold chain. These are refrigerated products shipped through ordinary parcel networks. If a package arrives warm, or has clearly been sitting outside, contact the provider before using it. Several of the companies we track advertise two to three day shipping, which is not the same as guaranteed temperature control.

Salt forms and additives. Not every form of these molecules is appropriate for compounding, and additives vary between pharmacies. Our guides on compounded vs FDA-approved GLP-1s and how compounding law actually works cover this in detail.

Who to call. Find out, before you order, which pharmacy is filling your prescription and how to reach the prescriber. Two of the providers we track name their pharmacies publicly. Most do not.

Practical things that help

None of this is medical advice, and none of it replaces a conversation with your prescriber. It is what people in this position commonly find useful.

• Eat smaller amounts more slowly, and stop earlier than feels natural. Overriding fullness is what turns mild nausea into vomiting.
• Prioritise protein and fibre in a smaller total volume of food, and expect constipation if you do not.
• Drink more water than you think you need. A lot of the fatigue and light-headedness people report is dehydration.
• Keep some resistance training in the week. Weight lost without it includes more muscle.
• Time the injection so the worst day lands where it disrupts you least.
• Keep a short log of dose, date and symptoms. If you need to escalate to a prescriber or dispute a charge, it is the only record you will have.
• Do not double up a missed dose, and do not adjust the schedule yourself.

If you need to pause, the billing does not pause with you

This is the part that catches people out, and it is the reason a review site has anything to say about side effects at all.

Side effects severe enough to need a pause do not stop the subscription. Embody asks for 72 hours' notice before a renewal. For WellMedr, use the updated cancellation guide and confirm the terms accepted with your order. SkinnyRx may charge a renewal up to five days early. Oak's terms state that once an order reaches the pharmacy it cannot be cancelled or refunded, even if you try to refuse the parcel. Direct Meds gives you 24 hours from placing an order and asks for 48 hours' notice before the next processing date.

So if you are stopping for medical reasons, cancel in writing the same day you make that decision, not when you next feel well enough to deal with it. Our cancellation guide has the notice window for each provider, and the refund comparison shows exactly when each window closes.

See refund and cancellation terms compared

Questions worth asking before you start

1. How do I reach a prescriber between shipments, how fast, and does it cost extra?
2. Which pharmacy is filling this, and what concentration is the vial?
3. Are my dosing instructions in milligrams or in syringe units?
4. What do I do if the package arrives warm?
5. If I need to stop for medical reasons mid-cycle, what happens to the charge?
6. Does the price change when my dose changes?

How we sourced this

Related reading

• How much weight can you expect to lose?
• Semaglutide vs tirzepatide
• What happens when you stop
• Compounded vs FDA-approved GLP-1s
• The full sixteen-provider comparison
• Refund and cancellation policies compared

Medical note