Can you have an unexpected allergic reaction to an old drug?

Old drugs. You can have a reaction to any drug at any time. The main thing that happens to old drugs is that they lose potency. Most drugs are more stable than we think, especially if kept dry and clean and in a stable temperature. So, yes, you can have a reaction, but you would probably have the same one if you took a new pill. Your immune system is more likely to have changed than the pill.