An MHRA update says treatment with quinine should only be considered when cramps severely disrupt sleep.
Quinine should only be used when cramps are very painful or frequent, other treatable causes of cramp are ruled out and symptoms are not alleviated by non-drug measures, such as passive stretching exercises, the MHRA said.
The agency said that although quinine has been used in the UK for the treatment of nocturnal leg cramps for many years and patient responses vary, its overall efficacy is modest.
Analysis suggests that, on average, quinine treatment leads to a reduction of one episode of cramping a week for each patient treated.
The MHRA also said patients should be monitored and treatment should be stopped if adverse effects occur in the early stages. After an initial four-week trial, treatment should be stopped if there is no benefit.
Adverse effects include tinnitus, impaired hearing, headache, nausea, disturbed vision, confusion, flushing and abdominal pain.
Thrombocytopenia, thought to be a hypersensitivity reaction, has also been reported. Patients should be told to stop treatment and consult a doctor if signs of thrombocytopenia occur, such as unexplained petechiae, bruising or bleeding, the MHRA said.