Yes, a dosage of 40mcg of clenbuterol has been shown to increase muscle mass in scientific studies, though the evidence is stronger in animal models than in humans.
Research in animals demonstrates a clear anabolic (muscle-building) effect at this dosage:
: A 2003 study found that administering 40ug/ml of clenbuterol to mice via drinking water for two weeks increased muscle fiber diameter by 26%. The same study noted a 219% increase in IGF-I mRNA (a key growth factor), suggesting this is a primary mechanism for the hypertrophy.
: Another study from 1994 showed that a single intraperitoneal injection of 40 micrograms of clenbuterol caused a rapid increase in cyclic AMP (a cellular signal for growth) in muscle tissue.
In human research, the evidence for muscle gain at 40mcg is more nuanced.
: A recent 2025 study found that prolonged oral clenbuterol significantly increased the abundance of KLHL41, a protein that stabilizes sarcomeres (the basic units of muscle). This upregulation was comparable to that seen with resistance training.
: A 2026 clinical trial (NCT04921306) used a daily dose of 40 µg of clenbuterol for four weeks. However, its primary goal was to measure improvements in muscle glucose uptake, not muscle mass. While it found a significant 13% increase in glucose uptake in the hamstring muscle, it did not report on overall lean mass gain as a primary outcome.
The 40mcg dose is considered a standard starting point in many contexts:
●It's the typical starting dose for men in bodybuilding cycles.
●It has been used as an initial dose in clinical trials for muscle-wasting conditions like Pompe disease.
●However, the "fat-burning" doses often used by athletes can be much higher, ranging from 120-160 mcg per day.
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