BACKGROUND: Magnesium sulphate is a neuroprotective agent that might improve 
outcome after aneurysmal subarachnoid haemorrhage by reducing the occurrence or 
improving the outcome of delayed cerebral ischaemia. We did a trial to test 
whether magnesium therapy improves outcome after aneurysmal subarachnoid 
haemorrhage.
METHODS: We did this phase 3 randomised, placebo-controlled trial in eight 
centres in Europe and South America. We randomly assigned (with 
computer-generated random numbers, with permuted blocks of four, stratified by 
centre) patients aged 18 years or older with an aneurysmal pattern of 
subarachnoid haemorrhage on brain imaging who were admitted to hospital within 4 
days of haemorrhage, to receive intravenous magnesium sulphate, 64 mmol/day, or 
placebo. We excluded patients with renal failure or bodyweight lower than 50 kg. 
Patients, treating physicians, and investigators assessing outcomes and 
analysing data were masked to the allocation. The primary outcome was poor 
outcome-defined as a score of 4-5 on the modified Rankin Scale-3 months after 
subarachnoid haemorrhage, or death. We analysed results by intention to treat. 
We also updated a previous meta-analysis of trials of magnesium treatment for 
aneurysmal subarachnoid haemorrhage. This study is registered with 
controlled-trials.com (ISRCTN 68742385) and the EU Clinical Trials Register 
(EudraCT 2006-003523-36).
FINDINGS: 1204 patients were enrolled, one of whom had his treatment allocation 
lost. 606 patients were assigned to the magnesium group (two lost to follow-up), 
597 to the placebo (one lost to follow-up). 158 patients (26·2%) had poor 
outcome in the magnesium group compared with 151 (25·3%) in the placebo group 
(risk ratio [RR] 1·03, 95% CI 0·85-1·25). Our updated meta-analysis of seven 
randomised trials involving 2047 patients shows that magnesium is not superior 
to placebo for reduction of poor outcome after aneurysmal subarachnoid 
haemorrhage (RR 0·96, 95% CI 0·86-1·08).
INTERPRETATION: Intravenous magnesium sulphate does not improve clinical outcome 
after aneurysmal subarachnoid haemorrhage, therefore routine administration of 
magnesium cannot be recommended.
FUNDING: Netherlands Heart Foundation, UK Medical Research Council.