How large of a change to the structure of an organic molecule has to be made for that molecule to have a major difference in its effect on a living system?

In theory - I believe - even the rearrangement of the molecule will change the property. For instance, there are two types of Aspirin, say Type 1 and Type 2. The Aspirin that you buy off shelf is Type 1, which is a pain killer in the correct doses. Type 2 Aspirin, which is just a slight rearrangement of Type 1 Aspirin, is a poison to human if given the same dose as Type 1, if that makes sense.