Lower numbers = closer to natural. Higher numbers = more enhanced.
Why this rating?
Clenbuterol is a beta-2 agonist that significantly increases metabolic rate and lipolysis through adrenergic receptor activation. A 2025 controlled human trial (Hostrup et al., J Physiol) confirmed it also produces a modest, non-hormonal muscle protein synthesis effect (0.91kg lean mass gain over a 2-week cycle) alongside a real drop in cardiorespiratory fitness (-7% VO2max). It does not touch anabolic hormones (no HPTA suppression, no PCT needed) and the muscle effect is small relative to steroids/SARMs. Not approved for human use and banned by WADA at all times (S1.2). Rated 6 to sit alongside cardarine: a significant, non-hormonal metabolic/performance enhancement beyond natural capacity, but capped below true anabolics since it does not build meaningful muscle.
Overview
A powerful Beta-2 agonist that increases basal metabolic rate and lipolysis. Originally developed as a bronchodilator, now widely used for rapid fat loss.
Important Warnings
- •Can cause cardiac hypertrophy with prolonged use
- •Not approved for human use in many countries
- •Can be dangerous for those with heart conditions
- •Not a benign "asthma inhaler": long half-life makes accidental overdose and prolonged tachycardia easy
- •Taurine + potassium/magnesium replacement is close to mandatory to control cramps
- •Documented case reports (not just a theoretical risk) include myocardial infarction, myocarditis and death in young, otherwise healthy bodybuilders using clenbuterol
Purpose & Use Cases
Rapid Fat Loss
Significantly increases metabolic rate for accelerated fat burning.
Muscle Preservation
A 2025 controlled human trial found clenbuterol directly increased muscle protein content and produced a modest (~0.9kg) lean mass gain via non-hormonal PKA/S6 signalling, on top of its anti-catabolic effect, though this signalling desensitizes within about two weeks.
Bronchodilation
Opens airways, improving breathing during cardio.
Benefits
- Significant increase in metabolic rate
- Enhanced lipolysis
- Mild anti-catabolic effects
- Improved cardiovascular performance
- Appetite suppression
Good to Know
It is NOT a "safe bronchodilator" fat burner
Clenbuterol is often rationalized as "just an asthma drug," but for physique use it is a potent, long-acting beta-2 agonist with a 35-48h half-life that hammers the cardiovascular and nervous systems. The fat-loss benefit is thermogenic, and the muscle effect is real but small, a 2025 controlled human trial measured only a modest ~0.9kg lean mass gain, nowhere near the dramatic repartitioning seen in livestock studies.
Beta-2 downregulation makes the effect fade fast
Chronic beta-2 stimulation downregulates the receptors within about 2-3 weeks, so the thermogenic effect fades. Chasing it by ramping the dose is exactly how people get into trouble. It multiplies tachycardia, tremor and cardiac strain without restoring fat loss. The fixes are cycling (classically 2 weeks on / 2 off) or ketotifen to restore receptor density, not more milligrams.
Cardiac risk is the real long-term concern
Animal research is mixed: some studies find clenbuterol-induced cardiac hypertrophy is largely "physiological" (no fibrosis) at moderate exposure, while others find myocyte necrosis/apoptosis appearing within hours at higher doses. In humans, case reports and a 2023 systematic review of 24 documented cases found real instances of myocardial infarction, myocarditis and dangerous arrhythmias in bodybuilders. This is why clenbuterol is treated as a short-cycle cutting tool, not something to run year-round, and why it is dangerous for anyone with an existing heart condition.
The cramps are an electrolyte/taurine problem
The classic clen muscle cramps come from taurine depletion plus potassium/magnesium shifts, not "dehydration" alone. Taurine 3-5g/day with potassium and magnesium is close to mandatory and resolves most cramping, see the support protocol.
WADA bans it as an anabolic agent, at ALL times
Because of its repartitioning/anabolic properties in livestock, clenbuterol is classified under S1.2 (Other Anabolic Agents), prohibited in and out of competition, unlike inhaled asthma beta-2 agonists (salbutamol) which sit under S3 with thresholds. It is also the source of famous "contaminated meat" doping positives, since it is used illegally as a growth promoter in cattle.
Dosage Guidelines
| Experience Level | Dosage Range |
|---|---|
| Beginner | 20 – 60 mcg/day |
| Intermediate | 60 – 100 mcg/day |
| Advanced | 100 – 120 mcg/day |
Start low and titrate up over the first several days. Use a 2 weeks on, 2 weeks off protocol due to receptor downregulation, a 2025 controlled human trial (80mcg/day) confirmed beta-2 signalling activation already declines within a single 2-week on-phase. A full cutting run is typically 2-3 repeated on/off blocks totaling 4-8 weeks; ketotifen (see support supplements) can be used to extend beyond that by restoring receptor sensitivity.
Side Effects
Tremors
very commonShaking hands and muscle tremors, especially at higher doses.
Start low and titrate up slowly. Usually diminishes with time.
Tachycardia
very commonElevated heart rate, sometimes significantly.
Monitor heart rate. Reduce dose if resting HR exceeds safe levels.
Serious Cardiac Events (rare)
rareA 2023 systematic review of 24 documented athlete cases (Kumari et al., Int J Legal Med) found supraventricular tachycardia, atrial fibrillation, myocardial injury/infarction, myocarditis and even death associated with clenbuterol use, including in young, previously healthy bodybuilders across a documented dose range of 20mcg up to gross overdoses.
Stay within established dose ranges, never combine with other stimulants, get any chest pain, palpitations, or breathlessness evaluated immediately, and avoid entirely with any pre-existing cardiac condition.
Anxiety
commonStimulant-induced anxiety and restlessness.
Start with very low doses; avoid caffeine.
Electrolyte Depletion
commonDepletes taurine and potassium, causing cramps.
Supplement with Taurine (3-5g daily) and Potassium.
Insomnia
commonDifficulty sleeping due to stimulant effects.
Take doses early in the day.
General Mitigation Strategies
Taurine (3-5g daily) and Potassium supplementation essential. Start with low dose and titrate up. 2 weeks on, 2 weeks off cycling to maintain receptor sensitivity.
Support Supplements
Ancillary supplements commonly run alongside Clenbuterol to manage side effects, support the target tissue, or fill nutrient demands it creates.
Taurine
KeyClenbuterol depletes intracellular taurine, which drives the muscle cramps and can worsen the headaches. Replacing it is the single most useful clen ancillary.
- Dose
- 3-5g/day
- Timing
- Split through the day; a dose before training helps cramping
Potassium + magnesium (electrolytes)
Beta-2 stimulation shifts potassium intracellularly and increases losses; low potassium/magnesium is the other half of the cramping and can add to palpitations. Replenish through diet plus supplementation.
- Dose
- Potassium from diet/electrolyte formulas; magnesium 300-400mg/day
- Timing
- Daily
Ketotifen (fumarate)
An H1-antihistamine that restores/upregulates beta-2 receptor density, reversing the tolerance that normally kills clen's effect after ~2-3 weeks. Lets some users run longer or continuous cycles instead of 2-on/2-off.
- Dose
- 1-2mg at night
- Timing
- Before bed (it is strongly sedating, that is a feature, it offsets clen's insomnia)
- When
- Optional, for extending cycle length. Sedation is significant; it does not reduce the cardiac risk of longer exposure.
Post Cycle Therapy (PCT)
Not hormonal. No PCT required.
How It Works
Clenbuterol stimulates beta-2 adrenergic receptors, increasing metabolic rate and thermogenesis. It enhances lipolysis by activating hormone-sensitive lipase, promoting the breakdown of stored fat. It also directly activates PKA/ribosomal S6 signalling in skeletal muscle, giving a modest non-hormonal boost to muscle protein synthesis, real in a 2025 controlled human trial, but small, and it desensitizes within about two weeks of continuous dosing (which is also why the thermogenic effect fades on a fixed schedule).
Fundamentals
Reference on the practices relevant to Clenbuterol: how they are done and where they go wrong. Not a recommendation to use it.
Detection Times
A controlled single 80mcg oral dose was detectable in urine for at least 7-10 days in a 2020 pharmacokinetic study; multi-week bodybuilding-style dosing, combined with the ~35-48h half-life, likely extends this further. WADA screens for it directly both in- and out-of-competition, and it is also the source of well-known "contaminated meat" doping positives from tainted livestock.
Legal Status
Not FDA-approved for human use in the USA and not a scheduled controlled substance; the only FDA-approved product (Ventipulmin oral syrup) is a veterinary bronchodilator for horses, illegal for use in any food-producing animal.
Legal status varies by country and changes over time. This is a general summary, not legal advice.
WADA Status
Clenbuterol is prohibited at all times. Although it is a beta-2 agonist, WADA lists it under S1.2 (Other Anabolic Agents) rather than S3 because of its anabolic/repartitioning properties. There is no inhaled-threshold allowance as there is for salbutamol. It is also a frequent source of contaminated-meat doping positives.
References
- Clenbuterol: Wikipedia (pharmacology, half-life, cardiac effects, livestock repartitioning & contaminated-meat doping positives, WADA anabolic classification)
- Hostrup et al. 2025, The Journal of Physiology, controlled human trial: oral clenbuterol (80mcg/day, 2-week cycles) increases lean mass/muscle protein but reduces VO2max and desensitizes beta-2 signalling within the cycle
- Kumari et al. 2023, International Journal of Legal Medicine: systematic review of 24 documented clenbuterol adverse-event cases in athletes (arrhythmia, myocardial injury, myocarditis, death)