Reactions are one of those D&D rules that everybody more or less understands right up until two things happen at once and somebody asks, “Wait, have I already used my Reaction?”
I have had that exact conversation more times than I can count, usually after someone makes an Opportunity Attack and then remembers they also wanted to cast Shield later in the round.
Unfortunately, you do have to choose.
A Reaction is basically something your character can do in response to a specific trigger. Unlike a normal Action or Bonus Action, it does not have to happen during your turn. In fact, most Reactions happen during someone else’s turn, although there is nothing stopping you from using one during your own if the correct trigger happens.
The important part is that you do not just decide to use a Reaction whenever you feel like it. Something has to give you one and tell you what triggers it.
The most common example is an Opportunity Attack. An enemy leaves your reach, which triggers the Reaction, and you can choose to make the attack.
Spells work the same way. Shield has a specific trigger. Counterspell has a specific trigger. Class features that use your Reaction will tell you exactly what has to happen before you can use them.
This sounds obvious, but I have definitely had players say something along the lines of “I use my Reaction to attack him” when absolutely nothing has happened that would allow them to do that.
A Reaction is not a smaller Action you keep in your pocket.
Once you use your Reaction, you cannot use another one until the start of your next turn.
And this is where the wording gets a little more important than simply saying you get “one Reaction per round,” because technically that is not how the rule works.
Let us say your turn begins, you do whatever you are doing, and then it ends. Later in that same round, an enemy runs away from you and you make an Opportunity Attack.
Your Reaction is now gone.
If another enemy attacks you before your next turn and you would normally be able to cast Shield, too bad. You already spent your Reaction stabbing the first guy.
Once the start of your next turn arrives, you get your Reaction back.
On the other hand, imagine you use a Reaction immediately before your turn begins. Your turn then starts, and your Reaction becomes available again straight away.
So, depending on initiative order and when things happen, you can sometimes use Reactions fairly close together. This is why thinking of them as strictly “one per round” can cause confusion. The actual rule is that after using one, you cannot use another until the start of your next turn.
You can also use a Reaction during your own turn.
That does not happen nearly as often, but it is completely legal. If something triggers a Reaction while you are taking your turn, you can react to it even though you are already using your Action, Bonus Action, movement, or whatever else you happen to be doing.
This can matter with spells in particular. You cast something, an enemy responds to it, and that response might trigger one of your own Reactions.
That said, if you use your Reaction during your own turn, you have still spent it. You do not finish your turn and magically get another one because everybody else has started acting. You are waiting until the start of your next turn like everyone else.
The Ready action also uses your Reaction, which is something players occasionally forget because the setup happens on your turn.
When you Ready something, you use your Action to decide what you want to do and what perceivable event will trigger it. Maybe you say, “If that goblin comes through the door, I shoot it.”
The goblin comes through the door later, and you can use your Reaction to do what you prepared.
However, this also means that if you already used your Reaction between readying the action and the trigger actually happening, you cannot use the readied action anymore. You spent the resource it needed.
Likewise, if the trigger never happens before the start of your next turn, you do not get to keep the prepared action indefinitely until the goblin eventually gets around to cooperating.
Readying a spell gets even more annoying because you cast the spell when you take the Ready action, hold onto its energy with Concentration, and then release it using your Reaction when the trigger occurs. If your Concentration breaks before that happens, the spell is gone.
This is one of those rules that feels like needless paperwork until someone tries to hold a spell for three rounds while also concentrating on something else, at which point I suddenly become very interested in paperwork.
The other thing I tend to remind players is that a Reaction is optional unless something specifically says otherwise.
An enemy leaving your reach does not force you to make an Opportunity Attack. If you think you might need Shield, Counterspell, or another more valuable Reaction later, you can simply let them walk away.
Sometimes hitting the fleeing goblin for six damage is worth it. Sometimes the enemy wizard standing twenty feet behind him should probably affect that decision.
As with most D&D rules, the exact wording of whatever is giving you the Reaction matters. Some happen immediately after their trigger. Some interrupt what is happening. Some have very specific timing written into the feature itself. When that happens, follow the individual rule rather than trying to apply one universal timing trick to every Reaction in the game.
So, the simple version is that Reactions happen when a specific trigger allows them, and you can use them on your turn or someone else’s. Once you spend yours, you do not get another until the start of your next turn.
Which means yes, if you used your Opportunity Attack already, you cannot cast Shield now.
I know. You really wanted to do both.
