Health professionals in the Forth Valley area have been issued with a warning to be vigilant after two cocaine users in Glasgow were treated for a rare and life-threatening condition associated with the use of the drug.

Methaemoglobinaemia is so rare that these two cases are the only ones registered in the UK linked with cocaine use.

Dr Henry Prempeh, NHS Forth Valley, consultant in public health, said in a statement: "This condition occurs when an increased quantity of the iron of haemoglobin is oxidised to the ferric form. Essentially it means an abnormality with a lack of oxygen in the blood.

"NHS Glasgow are uncertain as to the cause in these two cases and investigations are ongoing but it may be related to the consumption of an unusually large quantity of cocaine, or the presence of an unknown agent in the cocaine."

He added: "Apart from the fact that the cases are so rare, what is also of concern is that both cases occurred over a short five week period and both were in an area neighbouring ours. Both patients have now recovered and been discharged but they were very lucky - so serious is this condition that they may well have not survived."

Signs and symptoms of methemoglobinemia include shortness of breath, blue lips, headache, fatigue, exercise intolerance, dizziness and loss of consciousness.

Severe cases may lead to an irregular heart beat, seizures, coma and death.

Dr Prempeh urged: "If any cocaine users display any of these symptoms they should present to their nearest A&E, GP or health centre for prompt assessment and treatment for what is potentially a very serious condition."