What it is:A chronic, relapsing disease where excess adipose tissue, not just excess weight, drives metabolic, biomechanical, and psychosocial harm. The Obesity Medicine Association's definition matters clinically because it reframes obesity as a disease you treat long-term, not a willpower problem you lecture someone about.
The core problem:Chronic energy intake exceeding expenditure, but the body actively defends the higher weight once it's reached. Hormonal and neural appetite pathways shift after weight loss to push intake back up and expenditure down. That's why lifestyle change alone regains, and why the newest drugs (GLP-1/GIP agonists) that work directly on those appetite pathways outperform everything that came before them.
What you do about it:Lifestyle intervention is the foundation for everyone. Add pharmacotherapy at BMI ≥30, or ≥27 with a weight-related comorbidity. Reserve bariatric surgery for BMI ≥40, or ≥35 with significant comorbidity. Match the drug to the patient's comorbidities and contraindications, not just to whichever one is newest.
Language matters here more than in most disease states. Say "patient with obesity,"not "obese patient," and the term "morbid obesity" is retired. The OMA calls obesity a chronic, relapsing, multifactorial, neurobehavioral disease, and treating it as an identity or a moral failure is both wrong and bad medicine, since chronic disease means chronic treatment, not a 12-week fix.
BMI = weight (kg) / height (m)².It's the standard screening tool and what the FDA still uses for drug labeling and approval, but it does not replace clinical judgment. The AMA's 2023 policy update explicitly says BMI should be considered alongside waist circumference, percent body fat, and comorbidities, not used alone.
| BMI (kg/m²) | Category | Comorbidity risk |
|---|---|---|
| <18.5 | Underweight | Low (but other problems) |
| 18.5-24.9 | Normal weight | Average |
| 25.0-29.9 | Overweight | Increased |
| 30.0-34.9 | Obesity Class I | Moderate |
| 35.0-39.9 | Obesity Class II | Severe |
| ≥40 | Obesity Class III (extreme) | Very severe |
WC is measured at the narrowest point between the last rib and the top of the iliac crest, and it independently predicts disease risk even at a given BMI, because it's a proxy for visceral (intra-abdominal) fat, the kind that actually drives the metabolic syndrome. An elevated WC raises risk category even in someone whose BMI reads "normal."
| Population | Elevated WC threshold |
|---|---|
| Men | >40 in (102 cm) |
| Women | >35 in (89 cm) |
Waist-to-height ratio (WHtR) >0.5is considered a superior predictor of cardiovascular risk compared to WC alone, and it works consistently across sexes and ethnicities, which raw WC cutoffs don't do as well. Waist-to-hip ratio (WHR)flags increased metabolic risk above 0.90 in menand 0.85 in women. If a vignette gives you height, weight, waist, and hip, know which ratio the question is actually testing.
Obesity results from a sustained imbalance between energy intake and energy expenditure, but the etiology underneath that imbalance is multifactorial: genetics set both the tendency toward obesity and where fat gets distributed, while environment (food abundance, sedentary work, high-fat processed food, cultural and religious eating patterns) determines how much that genetic tendency gets expressed. Secondary causes exist too: Cushing syndrome, growth hormone deficiency, insulinoma, leptin deficiency, Prader-Willi syndrome, and psychiatric conditions like depression, binge-eating disorder, and schizophrenia.
Resting/basal metabolic rateis the single largest determinant of total energy expenditure and is relatively fixed per person. Physical activityis the other major component, and it's the most variable one, which is exactly why it's the lever both patients and clinicians reach for first, even though it's usually not enough on its own once significant weight has been gained.
White adipose tissuestores energy. Brown adipose tissueuncouples oxidative phosphorylation to burn energy as heat instead of storing it, and adrenergic stimulation activates lipolysis and increases energy expenditure in fat and skeletal muscle. This is the biologic basis for why sympathomimetic anorexiants (phentermine and its relatives) have some effect beyond simple appetite suppression.
GLP-1 (glucagon-like peptide-1) is an incretin hormone with three relevant actions: it increases glucose-dependent insulin secretion, inhibits gastric acid secretion and slows gastric emptying, and increases satiety by acting directly on the CNS appetite network. That third action is the one that makes GLP-1 receptor agonists weight-loss drugs and not just diabetes drugs: they work upstream, on the brain's hunger signal itself, rather than just blocking fat absorption or stimulating the CNS nonspecifically the way older agents do.
Obesity independently raises all-cause mortality, and central (visceral) obesityspecifically drives hypertension, dyslipidemia, type 2 diabetes, and cardiovascular disease, the cluster sometimes called metabolic syndrome. This is why the current treatment framework is a "complication-centric approach": the goal isn't a preset number on the scale, it's ameliorating the weight-related complications the patient actually has and improving their health and quality of life.
| Category | Examples |
|---|---|
| Metabolic | Type 2 diabetes, hyperlipidemia, metabolic syndrome, MASLD (metabolic dysfunction-associated steatotic liver disease) |
| Cardiovascular | Hypertension, coronary artery disease |
| Mental health | Anxiety, depression, insomnia |
| Mechanical / other | Obstructive sleep apnea, osteoarthritis |
These aren't just a checklist, they're the answer to "does this patient qualify for pharmacotherapy at BMI 27-29.9?"and they're also what you monitor to know if treatment is actually working, independent of the number on the scale.
Before you ever discuss weight-loss pharmacotherapy, review the patient's existing medication list. Swapping a weight-promoting agent for a weight-neutral or weight-losing alternative in the same class is often the highest-leverage move available, and it's free.
| Class | Weight-promoting examples |
|---|---|
| Anticonvulsants | Gabapentin, carbamazepine, valproic acid |
| Antidepressants | Most SSRIs (fluoxetine is the exception), TCAs, mirtazapine |
| Antipsychotics / mood stabilizers | Lithium, risperidone, olanzapine, clozapine, quetiapine |
| Antihyperglycemics | Insulin, thiazolidinediones, sulfonylureas, meglitinides |
| Beta blockers | Metoprolol, atenolol, propranolol |
| Steroid hormones | Corticosteroids, hormonal contraceptives, testosterone |
Give a patient a med list and ask which drugs contribute to weight gain versus weight loss. Insulin, atypical antipsychotics, and mirtazapine push weight up.Metformin, bupropion, and topiramateare weight-neutral to weight-losing and can be strategic substitutions or additions when a diabetic or depressed patient with obesity needs a medication anyway. Bupropion is a classic "kills two birds" choice when depression and obesity coexist, since it's also half of Contrave.
The organizing principle of modern obesity guidelines (AACE, ADA, VA/DoD, NICE) is the complication-centric approach: assess the presence and severity of weight-related complications first, then let that severity set the intensity of treatment. A patient with BMI 31 and no comorbidities is treated differently than a patient with BMI 31 and type 2 diabetes plus OSA, even though their BMI is identical.
| Intervention | Who qualifies |
|---|---|
| Lifestyle intervention | Everyone with overweight or obesity - the cornerstone for all tiers |
| Pharmacotherapy | BMI ≥30, or BMI ≥27 with ≥1 weight-related comorbidity, in a motivated patient who hasn't achieved/sustained loss with lifestyle change alone |
| Bariatric surgery | BMI ≥40, or BMI >35 with significant comorbidity (hypertension, T2DM, OSA) |
This is the single most tested operational rule in this chapter. Guidelines recommend discontinuing weight-loss pharmacotherapy after 3 monthsif the patient hasn't lost sufficient weight or is having significant adverse effects. Most individual agents have their own specific efficacy checkpoint at 12 weeks (Qsymia, Contrave, setmelanotide: discontinue if <5% weight loss)and liraglutide checks at 16 weeks (discontinue if <4% loss). Don't leave a patient on a drug that isn't working just because they tolerate it.
Comprehensive lifestyle intervention (diet, physical activity, behavioral counseling) is the cornerstone at every tier of obesity treatment, including in patients who also get a drug or surgery. Adherence to a low-calorie diet plus exercise plus in-person behavioral counseling produces an average of 8 kg (17.6 lb) weight loss over 6 months, which is the benchmark pharmacotherapy is layered on top of, not a replacement for it.
| Component | Target |
|---|---|
| Caloric deficit | 500-750 kcal/day deficit, OR estimated intake of 1200 kcal (women) to 1500 kcal (men) |
| Diet quality | Minimize ultra-processed, energy-dense foods; focus on nutrient density (AACE); the Plate Method is a patient-friendly ADA tool |
| Aerobic activity | 150 min/week moderate intensity, spread across ≥3 days/week |
| Resistance training | 2-3x/week to preserve lean muscle mass during weight loss |
The body mounts a maladaptive response after weight lossthat promotes weight regain, appetite hormones shift to increase hunger and metabolic rate drops more than predicted by the new, smaller body size. This is exactly why obesity is framed as a chronic relapsing disease rather than something "cured" by a single successful diet, and it's the biologic rationale for continuing pharmacotherapy long-term rather than stopping once a goal weight is hit.
long-termagents are approved for chronic, ongoing use. short-termagents are approved only for short courses (generally under 12 weeks, not to exceed 6 months) as an adjunct, not a chronic therapy.
| Class / Drug | Dosing | 1-yr weight loss above diet/exercise |
|---|---|---|
| Lipase inhibitor | ||
| Orlistat (Xenical, Rx) | 120 mg TID with fat-containing meals | 2.9-3.4 kg (~6%) long-term |
| Orlistat (Alli, OTC) | 60 mg TID with fat-containing meals | - |
| Phentermine-topiramate ER (C-IV) | ||
| Qsymia | 3.75/23 mg daily x14 days → 7.5/46 mg daily; max 15/92 mg daily | 6.6-8.6 kg (~9.5%) long-term |
| Naltrexone-bupropion | ||
| Contrave | Weekly step-up over 4 weeks to 16/180 mg BID (see titration below) | 4.9 kg (~5.5%) long-term |
| GLP-1 receptor agonists | ||
| Liraglutide (Saxenda) | 0.6 mg SC daily, increase by 0.6 mg weekly to 3 mg daily | 5.2 kg (~6.5%) long-term |
| Semaglutide SC (Wegovy) | 0.25 → 0.5 → 1 → 1.7 → 2.4 mg SC once weekly (4 wk steps) | 15.5 kg (~15.5%), 68-wk data long-term |
| Semaglutide oral (Wegovy tablet) | Titrate to 25 mg once daily, empty stomach, ≤4 oz water, 30 min before food/other oral meds | ~13.5% |
| Dual GIP/GLP-1 receptor agonist | ||
| Tirzepatide (Zepbound) | 2.5 mg SC weekly, titrate through 5 / 7.5 / 10 / 12.5 to 15 mg weekly | ~21% (highest of the class) long-term |
| Melanocortin-4 receptor agonist (niche, genetic obesity) | ||
| Setmelanotide (Imcivree) | Adults/adolescents ≥12y: 2 mg SC daily, up-titrate to 3 mg if tolerated; ages 6-<12y start 1 mg daily | 23.1% (POMC/PCSK1 deficiency), 9.7% (LEPR deficiency) |
| Short-term noradrenergic agents (adjunct only) | ||
| Phentermine (Adipex-P, Lomaira) C-IV | 8 mg TID before meals, or 15-37.5 mg/day in 1-2 divided doses | - short-term |
| Phendimetrazine (Bontril) C-III | 17.5 mg 2-3x daily before meals, or ER 105 mg once daily | - short-term |
| Diethylpropion (Tenuate) C-IV | 25 mg TID before meals, or CR 75 mg once daily (not at bedtime) | - short-term |
From least to most weight loss: bupropion-naltrexone (~5.5%) < orlistat (~6%) < liraglutide (~6.5%) < phentermine-topiramate (~9.5%) < oral semaglutide (~13.5%) < SC semaglutide (~15.5%) < tirzepatide (~21%). Tirzepatide's edge comes from hitting two incretin receptors (GIP andGLP-1) instead of one, and it isn't meant to be combined with a GLP-1-only agonist.
Orlistat is a lipase inhibitor, originally semisynthetic from a natural product (lipstatin, from Streptomyces toxytricini). It irreversibly (per Stevens' pharmacology framing) inhibits gastric and pancreatic lipase, blocking roughly 30% of dietary fat absorption. Crucially, it is not systemically absorbed, so its side effects and benefits are entirely local to the gut, which is why it's one of the few agents considered safe enough for a nonprescription (Alli) formulation.
Oily stools/steatorrhea, flatulence with discharge, oily rectal leakage, fecal urgency and incontinence occur in up to 80%of patients on the prescription dose. They're mild-to-moderate and improve after 1-2 months, but patients who aren't warned in advance stop the drug immediately out of embarrassment. Counsel: increase dietary fiber, decrease fat intake, take a multivitamin, and give it time.
Interactions:reduces absorption of fat-soluble vitamins A, D, E, and K, plus cyclosporine, levothyroxine, warfarin, amiodarone, and antiretrovirals. Separate dosing of these from orlistat and supplement fat-soluble vitamins. Skip the dose entirely if a meal has no fat or is skipped.
A fixed-dose combination of a sympathomimetic and an antiseizure drug, C-IV controlled. Phenterminereduces appetite through CNS stimulation of the hypothalamus to release norepinephrine. Topiramate'sweight-loss mechanism is genuinely not well understood, it was discovered as a side effect in epilepsy patients, but suspected mechanisms include blocking voltage-dependent sodium channels, enhancing GABA activity, and weak carbonic anhydrase inhibition, all contributing to appetite suppression and increased satiety.
Titration is slow and deliberate: 3.75/23 mg for 14 days, then 7.5/46 mg, up through 11.25/69 mg to a max of 15/92 mg. Take the dose in the morningto avoid insomnia. Reduce the max dose in moderate/severe renal impairment or moderate hepatic impairment to 7.5/46 mg.
Contraindications:pregnancy (embryo-fetal toxicity, REMS program restricts distribution), glaucoma, hyperthyroidism, and use within 14 days of an MAOI. Common effects: paresthesia, dysgeusia, dry mouth, constipation, insomnia, increased heart rate.
Bupropioninhibits dopamine and norepinephrine reuptake; naltrexoneis a pure opioid antagonist. Together they're thought to synergistically activate and sustain activation of hypothalamic POMC neurons, which naltrexone alone would blunt via opioid-mediated negative feedback.
| Week | Dose |
|---|---|
| 1 | 8/90 mg (1 tablet) once daily, morning |
| 2 | 8/90 mg twice daily (morning + evening) |
| 3 | 16/180 mg morning + 8/90 mg evening |
| 4+ | 16/180 mg (2 tablets) twice daily - maintenance |
Seizure disorder or anything that lowers seizure threshold (bupropion), history of anorexia or bulimia nervosa, uncontrolled hypertension, current opioid use or chronic opioid therapy (naltrexone will precipitate withdrawal), and use within 14 days of an MAOI. Also carries a suicidality warning shared with other bupropion products.
Counseling:do not take with a high-fat meal (increases exposure and seizure risk). Reduce max dose in moderate/severe renal impairment or hepatic impairment.
All members of this class share the same three actions: increase glucose-dependent insulin secretion, slow gastric emptying and inhibit gastric acid secretion, and increase satiety centrally. That's also why they share the same GI side-effect profile (nausea, vomiting, diarrhea or constipation, dyspepsia, abdominal pain) and the same rare-but-serious warnings.
| Drug | What makes it distinct |
|---|---|
| Liraglutide (Saxenda) | Once-daily; long-acting acylated GLP-1 analog, half-life ~13h (native GLP-1 is minutes) via a palmitic acid attached through a glutamate spacer, produced in yeast (S. cerevisiae) |
| Semaglutide (Wegovy) | Once-weekly SC, or once-daily oral tablet; resists DPP-4 degradation via amino acid substitutions, and a stearic diacid promotes albumin binding for an even longer half-life than liraglutide |
| Tirzepatide (Zepbound) | Once-weekly SC; dual GIP andGLP-1 receptor agonist, the added GIP activity is why it out-performs GLP-1-only agents; should not be combined with a GLP-1 receptor agonist |
Medullary thyroid carcinoma (MTC) and multiple endocrine neoplasia syndrome type 2 (MEN 2)- contraindicated with personal or family history of either. Discovered from rodent studies. Also watch for acute pancreatitis, acute gallbladder disease, acute kidney injury (usually secondary to GI losses/dehydration), and avoid in pregnancy.
Refrigerate all unopened pens/vials. Once in use: semaglutide injection 28 days at room temperature, oral tablets stay in original container; tirzepatide 21 days at room temperature; liraglutide 30 days at room temperature.
Phentermine, phendimetrazine, and diethylpropion are sympathomimetic amines: they release norepinephrine from adrenergic storage vesicles and block its reuptake. Their exact appetite-suppressing mechanism is secondary to general CNS stimulation, and it isn't fully characterized. All are Schedule III-IV controlled substances (less abuse potential than amphetamine, but real potential) and all develop tolerance, which is exactly why they're approved for short-term monotherapy only, generally under 12 weeks and never past 6 months. Continued use risks rebound weight gain once tolerance sets in, and abrupt discontinuation can cause withdrawal (depression, severe fatigue).
Prescribe the smallest quantitythat minimizes overdose potential, and don't combine two appetite suppressants together, since that compounds cardiovascular risk without added benefit.
A peptide analog of endogenous alpha-melanocyte stimulating hormone (α-MSH), acting as an MC4 receptor agonist. Unlike every other agent in this chapter, it's only indicated for patients with genetically confirmed or clinically suspected POMC, PCSK1, or LEPR deficiency, rare mutations that knock out the downstream signaling α-MSH normally provides. It won't work in garden-variety polygenic obesity because the receptor pathway it targets isn't the bottleneck there.
Notable side effects:injection site reactions, skin hyperpigmentation (from off-target melanocortin receptor activity), and spontaneous penile erections. Not recommended with moderate-severe kidney impairment. Discontinue if <5% weight loss at 12-16 weeks. It's also, by far, the most expensive agent in the class (~$19,800/month WAC), which matters when a payer asks why a cheaper GLP-1 wasn't tried first, the answer is that it wouldn't work on this mechanism.
Obesity pharmacology has a long list of drugs pulled from the market, and exam writers love asking whya specific agent was withdrawn, because the mechanism of the adverse effect usually maps directly back to the drug's mechanism of action.
| Drug | Status | Why withdrawn |
|---|---|---|
| Amphetamine | Banned from diet pills 1979 | The prototypical anorexiant; abuse/addiction potential |
| Ephedrine(Ma Huang) | Banned by FDA 2006 | Cardiovascular events; also a methamphetamine precursor, so sale is restricted |
| Fenfluramine(Pondimin, half of "Fen-Phen") | Withdrawn 1997 | Heart valve disease, via 5-HT2Breceptor stimulation causing inappropriate valve cell proliferation |
| Sibutramine(Meridia) | Withdrawn US-relevant markets 2010 | Cardiovascular risk (MI, stroke) with only minimal appetite-suppressing benefit; now a common undisclosed adulterant in "herbal" weight-loss supplements |
| Lorcaserin(Belviq) | Withdrawn 2020 | Increased cancer risk seen in long-term trial data; was originally developed as a selective5-HT2Cagonist specifically to avoid fenfluramine's valve problem |
| Rimonabant(Acomplia, CB1 antagonist) | Never approved in US; withdrawn in Europe 2008 | Increased risk of depression and suicidality |
Each drug's downfall traces to its mechanism: fenfluramine's serotonergic action hit heart valves(5-HT2B), sibutramine's adrenergic/serotonergic action hit the cardiovascular system, lorcaserin's serotonergic selectivity solved the valve problem but not a cancer signal, and rimonabant's CB1 blockade in the brain hit mood and suicidality. GLP-1/GIP agonists work through a different, gut-brain incretin axis, which is part of why their safety profile looks fundamentally different from this whole prior generation.
Dietary supplements aren't required to prove safety or efficacy before marketing. Many internet weight-loss products contain undisclosed appetite suppressants like ephedrine or sibutramine. 2,4-dinitrophenol (DNP)is a mitochondrial uncoupler still sold online despite never being approved, and it has caused documented deaths from hyperthermia. Berberine("nature's Ozempic") is a plant alkaloid under real study for modest GLP-1-releasing effects via bitter-taste receptors, but its effect size is mild and it needs high doses (>1000 mg/day) for weeks to show anything, nowhere near what patients are led to expect by the nickname.
Bariatric surgery, which reduces stomach volume or the absorptive surface of the alimentary tract, remains the most effectiveintervention available for obesity, more effective than any pharmacotherapy alone. It's reserved for BMI ≥40, or BMI >35 with significant comorbidity(hypertension, type 2 diabetes, obstructive sleep apnea). Implantable devices are an option for patients who don't qualify for surgery or decline it, and they still require concurrent diet and exercise adherence to work.
| Procedure | Basic concept |
|---|---|
| Sleeve gastrectomy | Removes most of the stomach, leaving a narrow tube - restrictive |
| Roux-en-Y gastric bypass (RYGB) | Small stomach pouch bypasses most of the stomach and proximal small bowel - restrictive + malabsorptive |
| Adjustable gastric band (AGB) | Adjustable band around the upper stomach - purely restrictive, less commonly used now |
| Biliopancreatic diversion with duodenal switch (BPD/DS) | Sleeve gastrectomy plus a long intestinal bypass - most malabsorptive, most weight loss, most nutritional risk |
| Single anastomosis duodeno-ileal bypass with sleeve (SADI-S) | Simplified single-connection variant of BPD/DS |
The more malabsorptive the procedure, the bigger the deficiency risk. Watch for fat-soluble vitamins (A, D, E, K), B1 (thiamine, urgent - can cause Wernicke encephalopathy), B12, folate, iron, zinc, copper, selenium, and calcium. Lifelong multivitamin and targeted supplementation is standard of care, and pharmacokinetics of oral medications can also change post-op (altered absorption surface, faster gastric emptying), which is worth a second look any time a post-bariatric patient's chronic meds seem to be under- or over-performing.
| Parameter | When | Watching for |
|---|---|---|
| Weight, BMI, WC | Monthly for the first 3 months, then every 3 months | Trajectory toward or away from the drug-specific efficacy checkpoint |
| Efficacy checkpoint | 12 weeks (Qsymia, Contrave, setmelanotide) or 16 weeks (liraglutide) | <5% (or <4% for liraglutide) weight loss → discontinue, it isn't going to work for this patient |
| Blood pressure | Every encounter | Improvement with weight loss; new hypotension with sympathomimetics |
| Tolerability / GI symptoms | Every encounter, especially during GLP-1/GIP titration | Nausea/vomiting severe enough to threaten adherence - slow the titration before abandoning the drug |
| Blood glucose (diabetic patients) | Weekly self-monitoring for 1-2 months after starting or adjusting therapy | Hypoglycemia as weight loss and improved insulin sensitivity reduce insulin/sulfonylurea requirements |
| Lipids / blood pressure endpoints | Periodically in patients with hyperlipidemia or hypertension | Whether weight loss is translating into the actual comorbidity benefit, not just a lighter scale reading |
| Mental health screen | Periodically, especially with any CNS-active agent | Mood changes, suicidal ideation (bupropion-naltrexone, historically rimonabant) |