Homeopathic medicines are said to have no side-effects; this is considered a major selling point. It is less often noticed that this is because they have no effects at all. This is why you can't overdose on homeopathic remedies. (I wouldn't try this myself, but see here.)
The difference between effects and side-effects is not a difference in the effect, but in our interests and aims. Throw a stone in a pond. What are the effects and what the side effects?
The stone hits the water. This makes a noise; there is a splash; there are ripples; the bird is startled and takes flight, and on and on. Maybe my purpose in throwing the stone is to make it skip; this is the effect at which I aim; relative to this aim, all the rest are just side-effects.