Bab 524: Spinning The Wheel.

Witches were always known for creating substances and potions. While potions were always liquid, substances were gooey and sticky.

Naturally, witches never aim to concoct substances since potions had five times better the effect than a substance. 

For example, when Felix used the eagle wings substance during the bus kidnapping attempt.

The substance applied directly on his shoulder blades, giving off grey wings. When its effect was gone, he was left with an itchy feeling like his skin was being eaten by ants.

That was a side effect that most substances had since they were an imperfect version of the potion that was meant to be created!

That's why he didn't feel any side effects when he drunk the angel wings potion and obtained those wings.

This didn't mean that potions didn't have any side effects whatsoever, it simply meant that it was highly likely for substances to have them due to their imperfect status.