What causes a sudden allergic reaction to a medication that has been taken for many years?

Many Risk Factors: Allergic reactions generally require previous exposure to the drug in question. There are many risk factors for developing an allergy to a medication including higher dose, IV administration, frequent repetitive courses, female gender, age, genetics, etc. Technically you can develop an allergy at any age. See Allergist to confirm allergy and to rule out a drug side effect or intolerance.

Uncertain: Nobody knows why for sure but even though one may be taken the same medication, one's genes do change constantly and thus the genetic evolution may alter your immune response to the drug. Another possibility is that during an infection, particularly viral, one's immune response is heightened making one more likely to become allergic to a drug. These are just some of my thoughts yet to be proven.