How do GLP-1 medications work for weight loss?
Medications based on GLP-1 receptor agonists represent a revolution in the medical treatment of obesity and overweight. These drugs mimic the action of the natural hormone glucagon-like peptide-1, which is produced in the gut after each meal. By activating GLP-1 receptors in the brain and digestive system, these medications produce a significant reduction in appetite, increase the feeling of fullness after eating, slow stomach emptying, and improve blood sugar regulation.
The net result of these combined effects is that people treated with GLP-1 agonists experience a natural and sustained reduction in caloric intake without the constant feeling of deprivation that characterizes conventional restrictive diets. Patients typically describe simply feeling less hungry, becoming full with smaller portions, and losing interest in the high-calorie, ultra-processed foods that they previously found irresistible.
Mounjaro (tirzepatide): the dual agonist
Mounjaro, developed by Eli Lilly, contains tirzepatide as its active ingredient and stands out from other medications in this category by being a dual agonist that simultaneously activates GIP and GLP-1 receptors. This dual mechanism of action has demonstrated superior efficacy in body weight reduction compared to single GLP-1 agonists in the SURMOUNT clinical trial program.
At the highest dose of 15 mg administered weekly, participants in the SURMOUNT-1 study achieved an average weight loss of 22.5 percent of initial body weight over 72 weeks of treatment. Mounjaro is administered using the pre-filled KwikPen, a device that uses a click system to dose the medication with precision. Available concentrations range from 2.5 mg to 15 mg, with a progressive titration schedule that starts at the lowest dose and gradually increases every four weeks.
Semaglutide: Ozempic and Wegovy
Semaglutide is a single GLP-1 receptor agonist marketed by Novo Nordisk under two different brand names depending on the indication: Ozempic for type 2 diabetes and Wegovy for weight management. Although the active ingredient is the same, the doses and approved indications differ between the two products.
Ozempic is available in doses of 0.25 mg, 0.5 mg, 1 mg, and 2 mg, with a primary indication for glycemic control in type 2 diabetes. Wegovy offers doses of 0.25 mg, 0.5 mg, 1 mg, 1.7 mg, and 2.4 mg, with a specific indication for chronic weight management in adults with obesity or overweight with comorbidities. In the STEP-1 trial, Wegovy 2.4 mg demonstrated an average weight loss of 14.9 percent of body weight over 68 weeks, a clinically significant result although lower than that obtained with tirzepatide for the same indication.
Other medications in development and emerging
The field of incretin-based weight loss medications is rapidly expanding. Several pharmaceutical companies are developing new agonists with even more complex mechanisms of action. Among the most promising are triple agonists that combine activation of GIP, GLP-1, and glucagon receptors, such as retatrutide from Eli Lilly, which in phase 2 clinical trials has demonstrated weight losses of up to 24 percent of body weight.
Oral formulations of these medications that would eliminate the need for injections are also in development. Oral semaglutide already exists as Rybelsus for diabetes, and both Eli Lilly and Novo Nordisk are researching high-dose oral versions specifically for weight management. These advances could significantly expand access to these treatments in the coming years.
Common side effects of GLP-1 agonists
The most frequent side effects of all GLP-1 agonists are gastrointestinal in nature and are usually temporary. Nausea affects approximately 30 to 50 percent of patients during the first weeks of treatment but decreases considerably over time and with each titration level. Constipation, diarrhea, bloating, and occasional vomiting are also common at the start.
Effective strategies to minimize these effects include eating smaller, more frequent portions, avoiding very fatty or processed foods, staying well hydrated, and respecting the recommended titration timelines without rushing to reach the maximum dose. In most cases, side effects are mild to moderate and do not require treatment discontinuation. If symptoms are severe or persistent, your doctor may adjust the titration speed or consider complementary pharmacological strategies.
Who is a candidate for these treatments?
Not all overweight patients are candidates for GLP-1 medications. European and Spanish clinical guidelines establish specific criteria for prescribing these drugs for weight management. Generally, candidates are adults with a body mass index (BMI) of 30 kg/m2 or higher, or with a BMI of 27 kg/m2 or higher who have at least one weight-related comorbidity, such as type 2 diabetes, hypertension, dyslipidemia, or sleep apnea.
Before starting treatment, your doctor will perform a comprehensive evaluation that includes blood tests to rule out endocrine causes of overweight (such as hypothyroidism), assessment of kidney and liver function, history of pancreatitis, personal or family history of medullary thyroid carcinoma, and an evaluation of psychiatric history. GLP-1 medications are contraindicated during pregnancy and breastfeeding, and are generally not prescribed to individuals under 18 years old for the obesity indication.
What to expect during treatment
The first visible results usually appear between the second and fourth week of treatment. Initial weight loss can be especially rapid, as it includes the reduction of fluid retention along with decreased caloric intake. From the second month onward, the loss stabilizes at a more sustained pace of between 0.5 and 1 kilogram per week on average, with significant individual variations.
One aspect that many patients report is the change in their relationship with food. Beyond the simple reduction in appetite, many describe a decrease in so-called food noise: those constant thoughts about food that accompany many people with overweight or obesity. This reduction in the compulsive desire to eat allows for more conscious food decisions and enjoying food without the urgency that previously dominated.
During dose increases, it is common for gastrointestinal side effects to temporarily reappear. Tolerance to each new dose level usually develops within one to two weeks. If the effects are particularly bothersome, your doctor may decide to slow the titration, keeping you at an intermediate dose for longer before increasing.
The role of physical activity
Although GLP-1 medications can produce significant weight loss even without changes in exercise, regular physical activity substantially improves the quality of results. Without exercise, a portion of the weight lost corresponds to lean muscle mass, which over time can reduce basal metabolic rate and facilitate weight regain.
The combination of aerobic exercise (walking, swimming, cycling) with resistance training (weights, resistance bands, bodyweight exercises) is the most effective strategy. Resistance exercise stimulates muscle protein synthesis and counteracts the loss of lean mass associated with caloric restriction. It is not necessary to become an athlete: even 150 minutes per week of moderate activity, the standard WHO recommendation, makes a significant difference in final body composition.
Frequently asked questions
Do I need a prescription for these medications?
Yes, all GLP-1 medications mentioned in this article require a medical prescription. They are not sold over the counter and should not be acquired through unauthorized channels. Your doctor will evaluate whether you are a suitable candidate based on your body mass index, comorbidities, medical history, and other relevant clinical factors.
Are these medications safe long-term?
The available safety data are reassuring. Semaglutide has follow-up data of more than five years, and tirzepatide has several years of data from clinical trials and commercial use. Cardiovascular studies have demonstrated additional heart health benefits. However, as with any medication, regular medical monitoring is essential throughout treatment.
Will I regain weight if I stop the medication?
Studies indicate that most patients regain some of the lost weight when discontinuing treatment, especially if they have not consolidated sustainable changes in their lifestyle habits. For this reason, many professionals consider these medications as a chronic treatment that should be combined with permanent modifications to diet and exercise to maximize long-term retention of benefits.
Can I combine these medications with other treatments?
GLP-1 agonists can be combined with certain diabetes or obesity medications under strict medical supervision. However, they should not be combined with each other or with other medications in the same pharmacological class. Inform your doctor of all medications, supplements, and natural products you are taking before starting treatment.