Complications from weight loss injections: what they are and what to do

Step 1: Recognise the expected discomfort (and how long it should last)

There is a well-established pattern that most people on GLP-1 weight loss injections go through in the early weeks. Nausea is the most frequently reported experience, not overwhelming for everyone, but noticeable enough that it tends to prompt a search like this one. Vomiting, loose stools or constipation, reflux, excessive burping and a general sense of fatigue also crop up commonly, particularly after starting treatment or stepping up to the next dose.

The NHS describes these effects as a predictable consequence of how the medicine slows gastric emptying and adjusts appetite signalling. They are not a sign that something has gone wrong. For most people they are at their worst in the first few days after a new pen or a new dose and settle considerably within one to two weeks. Eating smaller portions, staying well hydrated and avoiding fatty or heavily seasoned food during those early days tends to reduce the intensity.

The practical rule is this: if nausea is unpleasant but manageable, and you are keeping fluids down, it is likely the expected adjustment phase. If you cannot keep any fluids down for more than 24 hours, or if symptoms are severe rather than merely uncomfortable, that shifts the picture.

The technique used when injecting also matters, correct rotation of sites and proper storage reduce injection-site reactions, one of the more avoidable complications.

Step 2: Spot the complications that need a call to your prescriber or GP

Beyond the expected GI adjustment, a smaller group of complications warrant medical input rather than patient-managed waiting. Persistent vomiting that leads to dehydration is near the top of that list. When the body cannot absorb fluids, the kidneys can come under strain, this is particularly relevant for people who take blood pressure or diabetes medicines alongside their injection, since dehydration can interact with those treatments.

Gallbladder problems are a recognised complication of rapid weight loss in general, and GLP-1 medicines are associated with an increased rate of gallbladder events in trial data. Symptoms to watch for include pain in the upper right abdomen, pain that moves to the right shoulder, and pain that develops after eating. A GP or 111 call is appropriate if this pattern emerges.

Injection-site reactions beyond mild redness (hard lumps that persist, signs of infection or significant bruising) also merit a conversation with your prescriber. Rotating injection sites (abdomen, thigh, upper arm) on a regular basis reduces this risk considerably. Using alcohol swabs to clean the site before injecting is a straightforward step that helps.

Mood changes, including low mood or, rarely, thoughts of self-harm, have been reported with GLP-1 medicines. These should be taken seriously and reported to your prescriber or GP promptly. If you are concerned about which injection type carries which risk profile, that is a fair and reasonable question to bring to your clinical consultation.

Step 3: Recognise the emergencies that require 999 or A&E

A small number of complications from weight loss injections are medical emergencies. Acute pancreatitis sits at the top of this list. In January 2026, the MHRA issued a formal Drug Safety Update reinforcing that acute pancreatitis is a known, infrequent but potentially serious risk with GLP-1 medicines. The symptom to act on without delay is severe, persistent stomach pain that may spread to the back, sometimes accompanied by vomiting. That pattern should prompt a call to 999 or an immediate visit to A&E, not a wait-and-see approach. You can report a suspected reaction like this through the MHRA Yellow Card scheme, which helps regulators monitor real-world safety signals.

Allergic reactions are rare but can be severe: swelling of the face, lips or throat, difficulty breathing, or a sudden widespread rash all require emergency care. Severe hypoglycaemia (low blood sugar) is more relevant for people on diabetes medicines alongside their injection, symptoms include confusion, shaking, sweating, and loss of consciousness.

People with a history of medullary thyroid carcinoma or multiple endocrine neoplasia type 2 should not use these medicines. These are contraindications explored during the clinical consultation, not conditions a patient needs to self-assess, which is precisely why the prescriber review exists before any treatment starts.

If you are weighing up your options, the eligibility criteria for weight loss injections page explains what a prescriber considers at the outset, including pre-existing conditions that affect the risk profile.

Step 4: How ongoing clinical oversight reduces complication risk

A significant share of the complications that occur in practice are linked to dose increases that happen too fast, or to treatment continuing after warning signs that were not acted upon. This is where the structure of clinical oversight makes a material difference. The NHS England guidance on weight management injections is explicit that ongoing monitoring and wraparound support are part of safe prescribing, not optional extras.

At nume, a prescriber reviews every repeat order before it is dispensed. A dose increase requires clinical evidence, it is not automatic. That review catches the cases where symptoms suggest the current dose needs more time, or where a new medication or condition has changed the picture. You can find more about how the process works on the about us page, and our clinical team is available for aftercare queries seven days a week.

There is also a cost dimension worth being honest about: some patients seek the cheapest injectable weight loss option online and end up with counterfeit pens or no clinical review process. Both of those choices raise complication risk substantially. A legitimate supply chain and a live prescriber involved at each step are what the safety record is built on.

For those who prefer not to inject at all, there are oral alternatives licensed for weight management that carry their own risk profiles, worth exploring with a prescriber before deciding.