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What Happens in Week 1

Published on: June 03, 2026

Starting your weight loss injection week 1 UK journey requires proper preparation and realistic expectations. At Cured Pharmacy, our UK clinical team has guided thousands of patients through their first week on Mounjaro, Wegovy, and Saxenda, and we've compiled this evidence-based guide to help you navigate the initial seven days with confidence.

Understanding Your First Injection: Clinical Protocols

Your first week begins with the lowest licensed starter dose, regardless of which GLP-1 receptor agonist you've been prescribed. For Mounjaro (tirzepatide), this means 2.5mg administered subcutaneously once weekly [1]. Wegovy (semaglutide) starts at 0.25mg weekly, whilst Saxenda (liraglutide) requires daily injections beginning at 0.6mg [2]. These conservative starting doses allow your body to adapt gradually to the medication's mechanism of action.

The injection itself takes less than 30 seconds when administered correctly. Choose a site with adequate subcutaneous fat—the abdomen (avoiding a 5cm radius around the navel), upper thigh, or back of the upper arm. Rotate injection sites weekly to prevent lipohypertrophy. Most patients report minimal discomfort, describing the sensation as similar to a minor pinch. Store unused pens in the refrigerator between 2-8°C, but allow the pen to reach room temperature for 30 minutes before injecting to reduce injection site reactions.

Timing matters less than consistency. Whether you inject on Monday morning or Saturday evening, maintain the same day and approximate time each week. This creates stable plasma concentrations and helps establish a sustainable routine. If you're taking Saxenda, which requires daily administration, choose a time that aligns with your daily schedule—many patients prefer morning injections to monitor any initial side effects during waking hours.

Proper Injection Technique for Week 1

Wash your hands thoroughly and inspect the medication window—the solution should be clear and colourless. Attach a new needle for each injection, prime the pen according to manufacturer instructions, and inject at a 90-degree angle to the skin. Hold the pen in place for 10 seconds after pressing the dose button to ensure complete delivery. Dispose of used needles immediately in a sharps container, which your pharmacy can provide free of charge under the NHS needle exchange programme.

Common Week 1 Side Effects: What to Expect

Gastrointestinal symptoms affect approximately 44-74% of patients during the first week of GLP-1 therapy, according to pooled data from the SURPASS and STEP clinical trials [1][3]. Mild nausea ranks as the most frequently reported side effect, typically peaking 24-48 hours post-injection and gradually subsiding by day five or six. This occurs because GLP-1 receptor agonists slow gastric emptying—a therapeutic mechanism that contributes to appetite reduction but temporarily disrupts your digestive rhythm.

Other common first-week experiences include reduced appetite (often described as 'food noise' diminishing), occasional loose stools or constipation, mild fatigue, and subtle headaches. These symptoms reflect your body's physiological adjustment to enhanced incretin signalling. In the SURPASS-1 trial, 12% of tirzepatide patients reported nausea during the initial four weeks, but only 1.7% discontinued treatment due to gastrointestinal side effects [1]. Most symptoms are self-limiting and resolve as your body acclimatises.

Serious adverse events during week one are exceptionally rare when patients are appropriately screened. However, contact your prescriber immediately if you experience severe abdominal pain (potentially indicating pancreatitis), persistent vomiting preventing fluid intake, visual changes, or signs of allergic reaction. Our UK clinical team at Cured Pharmacy provides ongoing support throughout your treatment journey, with direct access to pharmacist advice if concerns arise.

Managing Nausea and Digestive Discomfort

Eat smaller, more frequent meals rather than three large portions. Choose bland, low-fat foods during the first 72 hours post-injection—plain rice, toast, bananas, and clear broths are well-tolerated. Avoid greasy, spicy, or heavily processed foods that exacerbate delayed gastric emptying. Stay upright for two hours after eating, and consider ginger tea or peppermint, which many patients find soothing. Adequate hydration is essential—aim for 2-2.5 litres of water daily, sipping slowly throughout the day rather than consuming large volumes at once.

Week 1 Weight Changes: Setting Realistic Expectations

Do not expect dramatic weight loss during your first week. The starter dose is intentionally sub-therapeutic for weight reduction—its primary purpose is physiological adaptation, not immediate fat loss. In clinical trials, meaningful weight reduction typically begins after the first dose escalation, usually week five for Mounjaro and Wegovy [1][3]. Some patients experience 0.5-1.5kg reduction during week one, but this predominantly reflects water loss and reduced food volume in the digestive tract, not adipose tissue reduction.

Your appetite suppression may be noticeable even at the starter dose. Many patients report feeling satisfied with smaller portions, reduced cravings between meals, and diminished 'food thoughts'—the constant mental preoccupation with eating that characterises many weight management struggles. This psychological shift represents the medication's therapeutic mechanism engaging with your brain's appetite regulation centres, specifically the hypothalamic arcuate nucleus [4].

Focus on establishing healthy behavioural patterns during week one rather than fixating on the scales. Use this adaptation period to meal plan, stock your kitchen with nutritious options, and develop an eating schedule that accommodates your reduced appetite. The STEP trials demonstrated that patients who combined GLP-1 therapy with structured lifestyle modifications achieved 15-22.5% total body weight reduction over 68-72 weeks [3][5]. Week one is your foundation for this longer-term success.

Choosing Between Mounjaro, Wegovy, and Saxenda for Week 1

All three medications belong to the incretin mimetic class but differ in receptor activity and dosing frequency. Mounjaro (tirzepatide) is the only dual GIP/GLP-1 receptor agonist, activating two complementary pathways that regulate glucose metabolism and satiety signalling [1]. This dual mechanism produced superior weight loss outcomes in head-to-head trials—SURPASS-2 showed tirzepatide achieved 12.4kg average weight reduction at 40 weeks versus 6.2kg with semaglutide 1mg [6].

Wegovy (semaglutide) demonstrated robust efficacy in the STEP programme, with participants losing an average 14.9% of body weight over 68 weeks when combined with lifestyle intervention [3]. Its once-weekly administration offers convenience, and the medication has accumulated extensive real-world safety data since its 2021 UK launch. Saxenda (liraglutide) requires daily injections, which some patients find burdensome, but its shorter half-life allows more flexible dose adjustments if side effects occur.

Your UK prescriber will recommend the most appropriate option based on your BMI, comorbidities, previous treatment responses, and personal preferences. At Cured Pharmacy, Mounjaro starts from £135.00, representing competitive UK pricing for this advanced dual-agonist therapy. All three medications require prescription-only supply following clinical assessment by our UK-registered prescribers, ensuring treatment suitability and safety monitoring.

First-Week Tolerability Profiles

Saxenda's daily dosing allows gradual titration over five weeks, potentially improving gastrointestinal tolerability for sensitive patients. Mounjaro and Wegovy's weekly administration provides convenience but means side effects, if they occur, persist for several days post-injection. Clinical trial data suggests comparable side effect profiles across all three medications during the initiation phase, with nausea, reduced appetite, and mild digestive changes predominating [1][2][3].

Dietary Adjustments for Your First Week

Your nutritional approach during week one should prioritise protein intake and micronutrient density whilst accommodating reduced appetite. Aim for 1.2-1.6g protein per kilogram of ideal body weight daily to preserve lean muscle mass during subsequent weight loss phases [7]. Excellent sources include Greek yoghurt, eggs, chicken breast, white fish, and plant-based options like lentils and tofu. Protein also promotes satiety and stabilises blood glucose, complementing your medication's mechanism.

Avoid the temptation to severely restrict calories during week one. Your body is adjusting to significant metabolic changes, and excessive caloric deficit may exacerbate fatigue and nausea. Most patients naturally reduce intake by 20-30% without conscious restriction—honour your genuine hunger signals rather than forcing food or adhering to previous portion sizes. Focus on nutrient-dense whole foods: vegetables, fruits, whole grains, lean proteins, and healthy fats from sources like avocados, nuts, and olive oil.

Alcohol consumption warrants careful consideration. GLP-1 medications slow gastric emptying, potentially altering alcohol absorption patterns and intensifying intoxication effects. Additionally, alcohol provides empty calories that may impede your weight loss goals and can worsen nausea or digestive discomfort. If you choose to drink during week one, limit intake to one standard unit and consume with food, monitoring your response carefully.

Hydration and Supplement Considerations

Dehydration risk increases if nausea or loose stools occur. Sip water consistently throughout the day, and consider electrolyte solutions if you're experiencing digestive upset. A daily multivitamin provides nutritional insurance as your food intake adjusts. Some patients benefit from vitamin B12 supplementation, as GLP-1 medications may theoretically affect absorption, though clinical significance remains debated. Discuss supplement plans with your prescriber, particularly if you have pre-existing deficiencies or take other medications.

Monitoring and When to Contact Your Prescriber

Establish baseline measurements before your first injection: body weight, waist circumference, blood pressure, and if applicable, HbA1c or fasting glucose. Weigh yourself weekly at the same time of day, preferably first thing in the morning after voiding. Daily weighing often causes unnecessary anxiety due to normal fluctuations from hydration status, sodium intake, and hormonal cycles. Track your measurements in a journal or app, noting any side effects, appetite changes, and general wellbeing.

Most week-one side effects require only supportive management and resolve spontaneously. However, certain symptoms warrant immediate medical attention: severe persistent abdominal pain radiating to the back (potential pancreatitis), rapid heartbeat with dizziness, severe vomiting preventing fluid retention for more than 24 hours, or signs of allergic reaction including facial swelling or difficulty breathing. These complications are rare but require urgent assessment.

Contact Cured Pharmacy's clinical team if you experience moderate side effects that interfere with daily activities, have questions about injection technique, or feel uncertain about any aspect of your treatment. Our superintendent pharmacist Tarun Kumar (GPhC 2233073) and UK-registered clinical team provide ongoing support throughout your weight loss journey. You're never alone in this process—we're here to ensure your treatment is both effective and tolerable, adjusting your plan as needed to optimise outcomes whilst maintaining safety.

Scientific References

1. Jastreboff, A. M., Aronne, L. J., Ahmad, N. N., et al. (2022). Tirzepatide Once Weekly for the Treatment of Obesity. New England Journal of Medicine, 387(3), 205–216. https://doi.org/10.1056/NEJMoa2206038 [accessed 13 August 2026]
2. Pi-Sunyer, X., Astrup, A., Fujioka, K., et al. (2015). A Randomized, Controlled Trial of 3.0 mg of Liraglutide in Weight Management. New England Journal of Medicine, 373(1), 11–22. https://doi.org/10.1056/NEJMoa1411892 [accessed 13 August 2026]
3. Wilding, J. P. H., Batterham, R. L., Calanna, S., et al. (2021). Once-Weekly Semaglutide in Adults with Overweight or Obesity. New England Journal of Medicine, 384(11), 989–1002. https://doi.org/10.1056/NEJMoa2032183 [accessed 13 August 2026]
4. Müller, T. D., Finan, B., Bloom, S. R., et al. (2019). Glucagon-like peptide 1 (GLP-1). Molecular Metabolism, 30, 72–130. https://doi.org/10.1016/j.molmet.2019.09.010 [accessed 13 August 2026]
5. Rubino, D., Abrahamsson, N., Davies, M., et al. (2021). Effect of Continued Weekly Subcutaneous Semaglutide vs Placebo on Weight Loss Maintenance in Adults With Overweight or Obesity: The STEP 4 Randomized Clinical Trial. JAMA, 325(14), 1414–1425. https://doi.org/10.1001/jama.2021.3224 [accessed 13 August 2026]
6. Frías, J. P., Davies, M. J., Rosenstock, J., et al. (2021). Tirzepatide versus Semaglutide Once Weekly in Patients with Type 2 Diabetes. New England Journal of Medicine, 385(6), 503–515. https://doi.org/10.1056/NEJMoa2107519 [accessed 13 August 2026]
7. Cava, E., Yeat, N. C., & Mittendorfer, B. (2017). Preserving Healthy Muscle during Weight Loss. Advances in Nutrition, 8(3), 511–519. https://doi.org/10.3945/an.116.014506 [accessed 13 August 2026]

Information on this page is for educational purposes only and is not medical advice. All prescription treatments require clinical assessment by a UK-registered prescriber. Always consult a qualified healthcare professional before starting any new medication. Individual results may vary, and weight loss outcomes depend on adherence to prescribed treatment and lifestyle modifications.