Love potions have appeared a few times in the manga. The first was a potion created by Negi in order to help Asuna. The actual effect of the potion was that anyone of the opposite sex who looked at the person who drank the potion (which in this case, was unfortunately Negi) would instantly fall in love with them. A second love potion appeared as a form of chocolates, and worked internally, the person eating them would fall in love with the first person they saw (which, again, was unfortunately Negi). This effect, unlike the first potion, was more long-term and subtle in effect, possibly in order to convince the victim that the effect is the result of their own feelings, and reduce the chances of their use being detected. This is because love potions/magic are actually outlawed contraband in Western Magic societies because they modify and alter the free will of other people, and in the case of these love potions- the romantic feelings of the person. Because they can be used for malicious purposes, it is illegal to use or produce them. Negi's first attempt to make one was a poor grade form at best and luckily, he had not been found out and punished for it.