Study 2: Acute tolerance study Acute tolerance will be investigated in study 2. Eight studies will be completed using a constant infusion of NAC over 60 mins at a dose identified in study 1. Forearm vasodilatation and the level of histamine release will be measured every 10 mins. This study may help to explain why in the clinical setting NAC can often be safely reintroduced, without complication, following a reaction.