In League of Legends, it is possible to slow down an enemy champion’s movement. This can be achieved by using skills, item passives, and item actives as well as by using a specific summoner spell.
With these many different ways of slowing down an enemy, the mathematics of the effect gets a little complicated.
The biggest question regarding the different slow effects is if they stack or not. While some of the slowing effect sources can somehow -we will get to this- stack, some don’t. So let’s take a look and how they behave.
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Item Passives
This is the simplest category we will go through. Slow effects coming from item passives simply do not stack. Instead, only the item passive that provides the biggest slow effect will be active.
For example, if the Unique Passives of both the “Duskblade of Draktharr” and “Rylai’s Crystal Scepter” items are activated on an enemy champion, only the slowing effect of Duskblade of Draktharr will take effect.
This is because the Unique Passive of Duskblade of Draktharr provides a ninety nine percent slowing effect while the Rylai’s Crystal Scepter only provides thirty percent.
This is the case even if both -or more- of these items are used by just one champion or two different ones.
Skills and Item Actives

The common thing Skills and Item Actives share is that they have to be used purposefully in order to affect a champion, so for the sake of simplicity we will call them “Actives”.
The big news is that they stack! Well, not exactly though… Remember when I said some slowing effects “somehow stack”? This is what I was talking about. The slowing effects of Actives stack but some of them will be reduced. Let’s get into the mathematics of it.
When multiple Active slowing effects are applied to a champion, only the strongest one will have its full effect. The others will be reduced by sixty-five percent, meaning they will only be thirty-five percent effective.
So, let’s say a champion was affected by both the slowing effect of “Nasus”s “Wither” ability and “Randuin’s Omen”s Unique Active. The Wither ability of Nasus provides a thirty-five percent slowing effect and the Unique Active of Randuin’s Omen provides slowing of a whopping ninety-nine percent.
Since the slowing effect of Randuin’s Omen would be the highest, it would apply completely. On top of that, thirty-five percent of the Wither’s thirty-five percent slowing effect would be applied to the champion too.
That is until the slowing effect of Randuin’s Omen wears off. Confusing, right?
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Crest of Cinders Buff

One of the ways of applying slow on a champion is the Crest of Cinders buff which you probably know as the Red Buff or just Red.
Red Buff, among many of its effects, also has a slowing ((10 / 15 / 25% for melee attacks) ( 5 / 7.5 / 12.5% for ranged attacks) based on level) debuff that is applied after a basic attack for three seconds.
Although the slowing effect of the Red Buff stacks with other slowing effects, it does not stack on itself. If the effects of Red Buff are activated on an enemy two or more times in a row, the effect will just refresh and not stack.
The Exhaust Summoner’s Spell

Exhaust is a that along with reducing the damage of a champion by forty percent, slows them by thirty percent for three seconds.
Exactly like the Crest of Cinders Buff, Exhaust only stacks with other slowing effects and only refreshes if cast on an enemy twice.
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Conclusion
There are multiple ways to apply the slow effect on an enemy and whether or not they will stack is decided by what their source is.
Only the slowing effects from Skills and Active Passives stack and even then, all of them besides the strongest one are heavily reduced.