What is the primary side effect associated with dantrolene usage?

Dantrolene is primarily known for its role as a muscle relaxant, specifically used to treat conditions characterized by excessive muscle spasticity and rigidity, such as malignant hyperthermia and conditions involving muscle over-excitation. The primary side effect associated with dantrolene is muscle weakness, which occurs due to its mechanism of action. Dantrolene exerts its effect by inhibiting calcium release from the sarcoplasmic reticulum in skeletal muscles, leading to decreased muscle contraction. This inhibition, while beneficial for reducing spasticity, may also result in generalized muscle weakness, making it a prominent side effect in patients taking this medication.

Other potential side effects such as fatigue, headaches, and abdominal pain can occur but are less specific or prevalent compared to muscle weakness, which directly relates to dantrolene's pharmacological profile. Therefore, muscle weakness stands out as the primary and most significant side effect associated with dantrolene usage.