What I Wish I Knew Before Starting GLP-1

GLP-1 medications are started at a very low dose and escalated gradually over 4–8 weeks, specifically to minimise side effects. On semaglutide this is 0.25mg weekly for four weeks, then 0.5mg. On tirzepatide it is 2.5mg for four weeks, then 5mg. During this escalation phase, nausea, fatigue, and digestive discomfort are at their worst for most people.

The most important thing to know: this is temporary. The dose escalation phase is not representative of what the medication feels like long-term. Most people find that side effects improve significantly or disappear entirely once they stabilise at a consistent dose. People who stop during the escalation phase often quit just before the experience would have improved.

This is the most clinically significant thing most GLP-1 users do not know, and the one with the most lasting consequences. The SURMOUNT-1 DXA substudy — published in Diabetes, Obesity and Metabolism in February 2025 — measured body composition in 160 participants using tirzepatide via DEXA scan over 72 weeks. The result: approximately 25% of total weight lost was lean mass, and 75% was fat mass. This proportion held consistently across age groups and sexes.

To make this concrete: someone starting at 90kg who loses 18kg on GLP-1 therapy will lose approximately 4.5kg of muscle alongside 13.5kg of fat — without deliberate intervention. That 4.5kg of muscle represents a meaningful reduction in resting metabolic rate, physical function, and the structural reserve that protects bone density over time.

Critically, the 25% lean mass loss figure is from trials that included individualised lifestyle counselling. The research on what reduces lean mass loss further is unambiguous: resistance training 3–5 days per week and adequate protein from day one. A 2025 case series published in PMC documented participants who achieved lean mass loss of only 6.9% — and one who gained 2.5% lean mass — while losing 27–33% of body weight, through consistent resistance training and protein intake of 1.6–2.3g per kilogram of fat-free mass daily.

Most people starting GLP-1 therapy expect nausea as an unavoidable consequence of the medication. The reality is more nuanced. GLP-1 medications slow gastric emptying, which makes high-fat foods, large portions, and rich restaurant meals significantly more nauseating than they would otherwise be. A 2025 clinical consensus in Mayo Clinic Proceedings identified high-fat foods specifically as the primary dietary trigger for GLP-1 gastrointestinal side effects.

People who eat small, low-fat, protein-forward meals during the escalation phase experience dramatically less nausea than those who continue eating normally. This is not willpower — it is physiology. Fried foods, heavy cream sauces, and large restaurant portions will reliably worsen nausea on GLP-1 therapy. Grilled protein, steamed vegetables, and small portions will not. Knowing this before you start lets you prevent most of the discomfort rather than managing it after the fact.

GLP-1 medications are weekly injections, and the timing you choose affects your quality of life significantly. Side effects including nausea and fatigue tend to peak in the 4–12 hours after injection and are typically at their lowest 24–48 hours later. Most people find that injecting on a Friday or Saturday evening means they sleep through the worst of the initial side effects and feel better by the weekend. Injecting on a Monday morning means navigating the worst side effects during a working day.

Restaurant meals, social events, and physically demanding activities are best planned 24–48 hours after injection when side effect burden is lowest. Over time, once side effects diminish, injection timing becomes less critical — but during the escalation phase it makes a real difference to daily function.

GLP-1 medications are highly effective at suppressing appetite. Too effective, for many people. It is common for users to consume fewer than 800–1,000 calories per day during the escalation phase without realising it — simply because hunger signals are so suppressed that there is no prompt to eat. This level of restriction accelerates both muscle loss and metabolic adaptation, and creates nutritional deficiencies in calcium, vitamin D, iron, and B12 that can take months to correct.

The minimum recommended calorie intake during GLP-1 therapy is typically 1,200–1,400 calories for women and 1,400–1,600 for men, with protein as the non-negotiable priority. Many people need to eat by the clock rather than by hunger during the early months — setting reminders to eat rather than waiting to feel hungry.

This is one of the most important practical things to know and one of the least discussed. GLP-1 medications slow gastric emptying, which means food stays in your stomach significantly longer than normal. During general anaesthesia or deep sedation, this creates a risk of pulmonary aspiration — stomach contents entering the lungs — even if you have followed standard fasting guidelines. The FDA updated GLP-1 medication labels in late 2024 specifically to warn of this risk.

Every healthcare provider who may administer sedation or anaesthesia — including for dental procedures, colonoscopies, and elective surgeries — needs to know you are on a GLP-1 medication. Your anaesthesia team may recommend holding the medication before surgery, following modified fasting guidelines, or using ultrasound to assess stomach contents before induction. Do not assume this information is in your records. Tell them directly.

Five independent studies now confirm that GLP-1 therapy is associated with reductions in bone mineral density, primarily driven by the rapid weight loss rather than a direct drug effect. A 2025 AAOS study of nearly 150,000 adults found approximately a 30% increase in relative osteoporosis risk over five years. A 2025 Endocrine Society study found total hip bone density declined by 2.8% over 34 months in high-risk patients.

The good news is that bone loss is largely preventable with the same interventions that protect muscle: resistance training, adequate calcium (1,000–1,200mg daily), vitamin D (600–1,000 IU daily), and protein. A JAMA Network Open 2024 study found that combining exercise with GLP-1 therapy preserved bone density completely despite significant weight loss. People who are postmenopausal, over 65, or already have osteopenia should discuss a baseline DEXA scan with their provider before or early in treatment.

GLP-1 medications alter the way alcohol is processed in two ways. First, by slowing gastric emptying, they delay alcohol absorption — which means the same amount of alcohol produces higher peak blood alcohol concentrations than it did before treatment. Second, GLP-1 receptors exist in the brain’s reward pathways, and the medications appear to reduce the rewarding sensation of alcohol for many users. Clinically, this means that people on GLP-1 therapy often find that they feel drunk faster, feel worse the next day, and in some cases lose interest in alcohol altogether. This is generally considered a beneficial side effect — but it can be surprising if unexpected.

This is the most important long-term reality of GLP-1 therapy, and it is rarely discussed at the point of prescription. The STEP 1 extension trial found that participants regained approximately two-thirds of their lost weight within one year of stopping semaglutide. This is not a failure of willpower — it reflects the fact that the appetite-regulating mechanism the medication provides disappears when the medication stops. Obesity is a chronic condition and GLP-1 medications treat it chronically, in the same way that blood pressure medication treats hypertension chronically.

This does not mean the medication must be taken forever for everyone. But it does mean that the lifestyle infrastructure built during treatment — protein habits, resistance training, food choices, portion awareness — needs to be robust enough to partially sustain the results if and when the medication is stopped. People who build these habits during treatment have significantly better outcomes after stopping than those who rely on the medication alone.