Risks - The side effects of the allopurinol are very rare (less than 1 in 10,000 people) and include headache, stomach upset, drowsiness and anaemia. Having blood tests taken can cause some mild bruising. The flow mediated dilatation may cause temporary numbness. MRI scanning is very safe and does not use radiation but some may feel a bit closed in. The scanner is a bit noisy but you will be given ear protection which also plays music. Your kidney function will be assessed before the scan to ensure it is safe to give you the contrast agent described above.