What causes weakness on one side?

The affected area and speed of onset matter. A stroke can cause sudden weakness of the face, arm, or leg on one side, often with speech, vision, balance, or severe headache changes. A transient ischaemic attack can cause similar symptoms that improve, but it still needs urgent assessment. Bell's palsy mainly affects facial movement and may make one side of the face droop.

Less urgent causes can include a trapped nerve, migraine-related symptoms, inflammation, or an injury. A clinician may compare strength, sensation, reflexes, coordination, speech, and facial movement, then decide whether scans or other tests are needed.

Note the exact time the weakness began, whether it is worsening, and which parts of the body are affected. Do not drive yourself if the change was sudden. If symptoms are new, sudden, or accompanied by speech trouble, face drooping, vision change, or loss of balance, call emergency services immediately, even if they improve.

Sudden one-sided weakness is an emergency until stroke or a transient ischaemic attack has been excluded. Seek immediate help for new facial drooping, difficulty speaking or understanding, sudden vision loss, severe headache, confusion, collapse, or trouble walking. Do not wait for the symptom to settle or arrange a routine appointment.

After urgent causes have been assessed, ask what examination or scan was needed, whether the pattern fits Bell's palsy or another nerve problem, and what changes should trigger a return to emergency care. Share your medicines and any history of migraine, diabetes, or previous neurological symptoms.