GLP-1 medications have mixed effects on athletic performance that depend heavily on baseline body composition, the type of sport, and whether the patient optimizes nutrition during treatment. Positive effects for athletes with excess body fat: significant weight loss can dramatically improve power-to-weight ratio (important for running, cycling, climbing), reduce joint load, and improve cardiovascular efficiency. Potential negative effects: caloric restriction from appetite suppression can reduce muscle glycogen availability, impair high-intensity energy system performance, and cause lean mass loss if protein and resistance training are not optimized. GLP-1 is not a performance-enhancing drug for lean athletes — it is a weight management medication that benefits athletes who carry excess body fat above their performance-optimal composition.

GLP-1's effect on muscle mass in athletes depends almost entirely on the training and nutrition response. Without mitigation: GLP-1 causes approximately 25–40% of weight loss to come from lean mass (consistent with any caloric restriction). With optimized protocol: resistance training 3–4x/week + protein 1.6–2.0g/kg body weight can reduce lean mass loss to <10% of total weight lost, and in some cases produce genuine muscle gain during fat loss in athletes with favorable body composition. Athletes who are actively training have significantly better lean mass preservation than sedentary GLP-1 users — the anabolic stimulus from training partially counteracts the catabolic pressure of caloric restriction. Creatine monohydrate 3–5g/day is an evidence-based addition to preserve lean mass during GLP-1 treatment.

GLP-1's effects on endurance performance are complex: (1) Weight loss benefit — for most endurance sports, carrying 15–20% less body mass directly improves performance (running economy, VO2 max per kg, power-to-weight ratio); (2) Glycogen availability concern — GLP-1's appetite suppression can reduce carbohydrate intake; low muscle glycogen impairs high-intensity endurance efforts and interval training; (3) Gastric emptying effect — GLP-1 slows gastric emptying, which can impair absorption of sports nutrition products (gels, drinks) during prolonged exercise; timing of GLP-1 dose relative to competition may affect fueling; (4) Electrolyte management — reduced intake increases electrolyte depletion risk during exercise; (5) For most recreational endurance athletes carrying excess weight, the weight loss benefit substantially outweighs the nutrition management challenges.

Athlete-specific protocol for GLP-1 treatment: (1) Protein: increase to 1.6–2.0g/kg body weight (higher than general population target of 1.2–1.6g/kg); distribute across 4–5 meals or feedings; (2) Carbohydrates: do not eliminate — strategic carbohydrate consumption around training sessions maintains glycogen and supports intensity; time carbs before and after workouts; (3) Caloric floor: active athletes should not drop below 1,600–2,000 kcal/day depending on training volume — appetite suppression can inadvertently underfuel training; (4) Creatine monohydrate: 3–5g/day for muscle preservation and power output support; (5) Race/competition day: be aware that GLP-1 slows gastric emptying — sports nutrition products (gels, chews) may absorb more slowly; (6) Training load: maintain or increase resistance training; reduce high-intensity cardio volume temporarily during titration if performance suffers.

As of April 2026, GLP-1 medications (semaglutide, tirzepatide, liraglutide) are not on the World Anti-Doping Agency (WADA) Prohibited List. They are neither stimulants, anabolic agents, hormone agonists in the banned sense, nor performance-enhancing in the mechanisms WADA targets. Athletes at all competitive levels may use GLP-1 medications for legitimate medical indications (obesity, type 2 diabetes) without TUE (Therapeutic Use Exemption) requirements under current rules. This status could change if evidence of performance-enhancing effects accumulates in lean athletes — WADA monitors emerging substances — but no regulatory action has been indicated. Athletes in tested sports should confirm with their sport's governing body and WADA's current list before use.

Athletes most likely to benefit from GLP-1: (1) Recreational and masters athletes carrying 15–30 lbs above their performance-optimal weight — power-to-weight improvement translates directly to performance; (2) Athletes in weight-class sports (powerlifting, wrestling, MMA) who struggle with weight management between competitions — GLP-1 can facilitate sustained weight management rather than cyclical extreme cuts; (3) Endurance athletes (runners, cyclists, triathletes) where power-to-weight ratio is primary performance determinant and excess weight meaningfully impairs results; (4) Athletes with obesity-related comorbidities (sleep apnea, hypertension, pre-diabetes) where treatment improves training consistency and recovery. GLP-1 is unlikely to benefit lean, elite athletes at ideal competition weight — the medication has no ergogenic effect on the fundamental physiological parameters that determine performance in well-conditioned athletes.