No, there are drugs that do turn toxic after time. Acetaminophen-codeine (tylenol #3) causes liver damage after 3 months. The codeine reacts with the acetaminophen (APAP) and turns it toxic. This is not related to the liver damage that prolonged exposure to APAP causes.
I'm a health professional that deals with pain relief daily.

To all of you claiming chicken pox is not dangerous:

WRONG

It can be very dangerous. It is in fact lethal in some children. I lost my three year old sister to it. The autopsey found it in every organ in her body, including the brain. There were also many cases of lifetime disabilities caused by it.

That is not even talking about herpes zoster or the other effects it has on adults. Just becaus it isn't as virulent, damaging, or lethal as small pox, chicken pox is not something to take lightly.