We will take a structured 3-part approach to the research hypotheses. Studies 1 and 2 will aim to establish a dose-response curve and seek evidence of tachyphylaxis, while measuring the level of histamine release. The presence of tachyphylaxis may help to explain why, in the clinical setting, NAC can often be safely re-introduced following a reaction. In study 3, the investigators intend to conduct a 4-way randomised controlled crossover mechanistic study. This will examine the pharmacodynamic effect of NAC in both the presence and absence of histamine antagonists (antihistamines), and low- and high-dose paracetamol. This study will also enable investigation of a possible protective role of paracetamol.