hypersensitivity

n. excessive sensitivity; abnormal sensitivity to a substance which does not usually have a negative affect on the body (i.e. medication)

Hypersensitivity refers to undesirable (damaging, discomfort-producing and sometimes fatal) reactions produced by the normal immune system. Hypersensitivity reactions require a pre-sensitized (immune) state of the host. The four-group classification was expounded by P. H. G. Gell and Robin Coombs in 1963.
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Hypersensitivity /hy·per·sen·si·tiv·i·ty/ a state of altered reactivity in which the body reacts with an exaggerated immune response to what is [more]Hypersensitivity - Community and Resources

Noun
1. pathological sensitivity
(hypernym) sensitivity, predisposition
(hyponym) cryesthesia, cryaesthesia
2. extreme sensitivity
(hypernym) sensitivity, sensitiveness, sensibility

An exaggerated response by the immune system to a drug or other substance.