The bottle in question has a label around its neck that says "DRINK ME." It does not say anything further as to what will happen when Alice does so. It is notable that this bottle appears at the same time that Alice is wishing she could fold down like a telescope in order to be able to fit through a very tiny door she has spotted. Through this door, Alice can see beautiful gardens, but the door is barely the size of a mousehole, and Alice, of course, cannot fit through it.
Alice does not drink from the bottle immediately upon seeing it. A sensible child, she knows that sometimes things in bottles can be poisonous, but, finding that there is no marking on the bottle to indicate that this might be poison, she decides it is probably safe to drink. Alice then drinks from the bottle and experiences a curious feeling, which she quickly decides must be the feeling of being folded up like a telescope. Sure enough, she soon discovers that she has shrunk and is now small enough to walk through the tiny door.