Albuterol (Salbutamol)

Which albuterol

Albuterol and salbutamol are one molecule under two naming systems, the United States adopted name and the international nonproprietary name. PubChem returns a single compound record for either term: CID 2083, formula C13H21NO3, molecular weight 239.31, InChIKey NDAUXUAQIAJITI-UHFFFAOYSA-N. The FDA Global Substance Registration System carries the same structure under UNII QF8SVZ843E, with CAS 18559-94-9 marked primary and 35763-26-9 marked superseded, ChEMBL714, ATC code R03AC02, and a listing on the WHO Model List of Essential Medicines at section 25.1. The GSRS structure record is flagged RACEMIC.

Products contain the salt rather than the base, and the two registries disagree about which record its CAS belongs to. GSRS assigns CAS 51022-70-9 to the 2:1 hemisulfate under UNII 021SEF3731, empirical formula (C13H21NO3)2 with H2SO4, molecular weight 576.70, which the approved labelling writes as 576.71, and links that record to PubChem CID 39859. PubChem resolves the same CAS to CID 9884233, a 1:1 adduct at 337.39 g/mol titled salbutamol sulfate, whose synonym list also carries 2:1 naming, while CID 39859 carries the CAS 36519-31-0 instead. Only the 2:1 form corresponds to the substance in registered products, so a molecular weight near 337 quoted for a salt of this compound is describing a database record rather than a medicine.

The single-enantiomer form has a separate identity. Levalbuterol, the (R)-enantiomer, carries UNII EDN2NBH5SS, CAS 34391-04-3, PubChem CID 123600 and ChEMBL1002, with the same formula and mass as the racemate. GSRS assigns ChEMBL714 to both the racemic base and the sulfate, so a ChEMBL code alone does not separate them. Half of racemic albuterol is (S)-albuterol, which has its own disposition and its own published literature, reported separately from the (R)-enantiomer in the pharmacokinetic work cited below.

What the approved labelling establishes

Albuterol is an approved medicine, which distinguishes it from most compounds catalogued here. The indications section of current oral tablet labelling reads: relief of bronchospasm in adults and children six years of age and older with reversible obstructive airway disease. Nothing in that labelling concerns muscle mass, body composition, or performance. Drugs@FDA records inhalation aerosol approvals from August 1996 onward and a fixed combination of albuterol with budesonide approved under NDA 214070 in January 2023; oral tablets remain marketed under generic applications.

Label pharmacokinetics come from volunteer studies rather than from any secondary summary. Oral absorption is described as rapid, with mean steady-state peak and trough plasma concentrations of 14.8 and 8.6 ng/mL on a 4 mg tablet every six hours, maximum concentrations between two and three hours, and an elimination half-life of five to six hours. Analysis of urine from volunteers given tritiated albuterol at 4 to 10 mg orally recovered 65% to 90% of the dose over three days, most of it within the first 24 hours, of which 60% was metabolite. Faeces contained 4%.

Frequencies for the oral tablet are tabulated in the same document. Nervousness and tremor were each recorded in 20% of adults and children six and older, headache in 7%, palpitations and tachycardia in 5% each, muscle cramps in 3%, nausea in 2%. Preclinical statements on the label note that intravenous studies in rats found brain concentrations around 5% of plasma, with concentrations in the pineal and pituitary glands roughly 100 times those in whole brain.

What has been measured in human muscle

Martineau and colleagues published the first controlled human strength data in 1992. Twelve healthy men received a slow-release salbutamol preparation or placebo, with strength of several muscle groups measured before treatment and after 14 and 21 days. Quadriceps strength rose 12 plus or minus 3% at 14 days and stayed elevated at 21. Dominant-leg hamstring strength rose 22 plus or minus 6% at 21 days while the non-dominant side returned to baseline. Grip strength did not change. Body weight, skinfold thickness, lean body mass and limb circumferences did not change in either group.

Kissel and colleagues randomised 90 patients with facioscapulohumeral dystrophy to placebo or sustained-release albuterol at 8 or 16 mg twice daily for a year, with 84 completing. Composite maximum voluntary isometric contraction scores did not differ between groups, and neither did manual muscle testing. Grip improved in both treatment groups. Lean mass by dual-energy X-ray absorptiometry rose 1.57 plus or minus 1.71 kg in the high-dose group against 0.25 plus or minus 2.24 kg on placebo. Recorded side effects were cramps, tremors, insomnia and nervousness.

Two shorter studies measured power and maximal strength separately. Van Baak and colleagues gave 16 nonasthmatic men a single 4 mg oral dose in a crossover design and recorded knee extensor peak torque 4.4% higher and knee flexor 4.9% higher than placebo, with plasma lactate and potassium changed. Hostrup and colleagues randomised 20 elite male endurance athletes to 8 mg oral salbutamol or placebo in a parallel design, roughly ten per arm, and recorded Wingate peak power 4.1 plus or minus 1.7% higher acutely and 6.4 plus or minus 2.0% higher after two weeks in the salbutamol arm, with maximal voluntary contraction unchanged.

The 2026 resistance-training trial

Hostrup and colleagues reported the trial that most directly addresses why this compound circulates outside asthma care. Thirty healthy trained men, mean age 23, received oral salbutamol at 16 mg per day or placebo through an eleven-week supervised full-body resistance training programme of three sessions weekly; 26 completed. The primary outcome was change in lean mass, assessed by dual-energy X-ray absorptiometry, and lean mass increased 1.8 kg more in the salbutamol group than in placebo, with a 95% confidence interval of 0.5 to 3.1 kg.

The trial's other endpoints did not follow the lean mass result. Muscle capillary density and the activities of citrate synthase and 3-hydroxyacyl-CoA dehydrogenase, measured in vastus lateralis biopsies, decreased in the salbutamol group and did not change in placebo. Time to exhaustion during cardiopulmonary exercise testing did not change under salbutamol and rose 7% in placebo. Echocardiography recorded increased posterior, septal and relative wall thickness against placebo, while cardiac magnetic resonance imaging in the same participants showed no between-group difference in cardiac structure or function, left ventricular mass included.

Echocardiography and cardiac magnetic resonance imaging disagreed within the same trial of 26 completers, and both results were reported. The trial reported four sets of outcomes: lean mass, muscle oxidative capacity, cardiopulmonary fitness and cardiac imaging. Only the first favoured salbutamol. The imaging, cardiopulmonary and biopsy endpoints were secondary to the primary lean mass outcome. The authors framed their own findings as risk-benefit trade-offs and as support for restricting supratherapeutic salbutamol in sport.

Fat mass, metabolic rate, and one registered trial

Exactly one registered trial in the ClinicalTrials.gov database tested this compound against body composition and posted its results. NCT02740660, run at Pennington Biomedical Research Center, randomised 12 adolescents aged 12 to 17 with obesity, six per arm, to oral capsules containing albuterol 4 mg with caffeine 100 mg or placebo three times daily for eight weeks alongside family weight-management counselling. Fat mass changed by plus 0.97 kg with a standard deviation of 1.11 in the treatment arm against minus 0.21 kg, standard deviation 1.41, on placebo. Lean mass rose 1.34 against 0.88 kg, and body weight 2.57 against 1.05 kg.

Liu and colleagues published the wider programme behind that trial in 2015. It combined three different experimental systems. Caffeine and albuterol each enhanced lipolysis in cultured adipocytes. Acute administration raised resting metabolic rate in humans. The body composition result, increased lean mass gain with decreased fat mass gain, was obtained in rats over four to eight weeks without a change in food intake. The human and the body-composition arms are different experiments in different species, and the paper says so.

Thermogenic figures in circulation trace mostly to infusion studies. Hoeks and colleagues gave nine lean men three-hour infusions of a beta-1 agonist, salbutamol, or salbutamol with the lipolysis blocker acipimox; energy expenditure rose around 13% in all three conditions, and fat oxidation rose 19 plus or minus 7% under salbutamol alone. Straat and colleagues later reported that a single intravenous bolus increased brown adipose tissue glucose uptake on positron emission tomography relative to salbutamol given with propranolol, in a randomised double-blind crossover of young lean men; the number of participants is not stated in the retrievable record.

Rodent muscle work

Carter and Lynch ran the only head-to-head comparison of the two compounds located in any species. Fisher-344 rats aged 3 and 24 months received salbutamol at 1.03 mg per kg per 24 hours or clenbuterol at 600 micrograms per kg per 24 hours by implanted osmotic minipump for three weeks. Combined hindlimb muscle weight rose 19% with salbutamol and 28% with clenbuterol in young rats, and 19% and 25% in old rats. Carcass protein content rose 20% and 30% in young animals, 12% and 21% in old.

Soic-Vranic and colleagues tracked fibre types rather than mass. Male Wistar rats were divided into four groups: control (n = 10), salbutamol (n = 30), denervated (n = 30), and salbutamol after denervation (n = 30). Salbutamol was injected intraperitoneally at 0.3 mg per kg twice daily for two weeks, and muscles were examined 3, 6, 9, 12 and 14 days after treatment began. Hypertrophy was reported across all fibre types in healthy soleus, along with a higher percentage of type II fibres, 21%. In denervated muscle, type I fibre cross-sectional area was 28.2% larger than in untreated denervated muscle.

These are rodent findings, obtained under continuous minipump delivery in the first study and twice-daily injection in the second. The FDA-approved oral tablet labelling reports an elimination half-life of five to six hours and maximum plasma concentrations between two and three hours after dosing. The doses administered in these rodent studies are higher, per kilogram, than any dose that labelling describes. No study has compared the two delivery modes directly. The Carter and Lynch comparison measured hindlimb muscles and carcass composition, and reported no cardiac endpoints.

Doping control thresholds by route

Section S3 of the World Anti-Doping Agency 2026 Prohibited List bans all selective and non-selective beta-2 agonists including all optical isomers, and names salbutamol and levosalbutamol explicitly. The exception is inhaled salbutamol at a maximum of 1600 micrograms over 24 hours in divided doses not exceeding 600 micrograms over eight hours from any dose. Presence in urine above 1000 ng/mL is treated as an Adverse Analytical Finding unless the athlete demonstrates through a controlled pharmacokinetic study that it followed a permitted inhaled dose.

Whether that threshold does the job it is asked to do is disputed in the pharmacology literature. Heuberger and colleagues built a semi-physiological model from published parameters and simulated 1000 virtual subjects; 15.4% exceeded 1000 ng/mL one hour after a permitted inhaled dose, and the authors concluded that a single untimed urine sample cannot recover the administered dose. Courlet and colleagues, working from a model-based meta-analysis of 13 studies, reached a milder verdict: broadly supported, with genuine uncertainty for values near the threshold.

Enantiomer behaviour complicates the same measurement. Hostrup and colleagues sampled arterial plasma and vastus lateralis in 12 lean young men at 2.5 and 7 hours after 24 mg orally. Plasma concentrations of (S)-salbutamol ran roughly tenfold above (R)-salbutamol, reflecting faster elimination of the (R)-enantiomer, and muscle concentrations were likewise higher for (S). The muscle-to-plasma partition coefficient at 7 hours, however, was twofold higher for (R). Concentration-time profiles in muscle and in circulation were not the same shape.

What is not known

Human exposure in the published record tops out at 52 weeks, in patients with a muscular dystrophy rather than in healthy adults; the longest healthy-adult trial ran eleven weeks in 26 completers. No study has followed what happens to the recorded lean mass, the reduced oxidative enzyme activities or the echocardiographic wall thickness after treatment stops, so nothing establishes whether any of them persist or reverse. The disagreement between echocardiography and cardiac magnetic resonance within the 2026 trial is unresolved, and no larger imaging study exists to settle it. Sex representation is narrow: the resistance-training, sprint and infusion studies enrolled men only, and the single registered body-composition trial enrolled six girls and six boys aged 12 to 17. No trial has tested oral albuterol against body composition in adults without a co-administered stimulant. The cardiovascular risk estimate quoted here pools several beta-2 agonists in patients with obstructive airway disease and cannot be read as an albuterol-specific figure for healthy people. Albuterol is approved as a bronchodilator and appears on the WHO Model List of Essential Medicines; current FDA labelling carries no indication concerning muscle mass, fat mass or performance, and no non-US drug register was successfully queried for this page, so no worldwide statement about approved indications is made here. Systemic administration is prohibited in sport under WADA section S3. Material obtained outside a pharmacy supply chain has not been subject to the identity, purity or content controls that apply to registered products.

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