Why some people are not allergy to some medicine at first but later become allergic?

Source: Kingworld medicine

In general, the human body will not have allergic reaction if it does not take the medicine before, because the body does not have the antibody for that medicine. Once the body takes this medicine, the antibody will exist in the body and the allergic reaction may occur when take that medicine again. Besides, some allergic reaction is to some impurity, accessory, or additive in the medicine. Different manufacturers may have different manufacturing technique or production equipment, thus the impurity, accessory, and additive may be different. That’s how the situation not allergic at first but later happens.