# Can I drop concentration on a damaging spell before it affects an ally in combat? [duplicate]

If on my turn I cast a damage spell that requires concentration, can I keep it up on subsequent enemy turns then drop it before it damages an ally at the start of their turn?

For example, this is the initiative order in a round: my Wizard > Enemy Stirge > Ally Fighter. On a previous turn the Stirge has attached itself to the Fighter using Blood Drain. On my turn I cast Cloud of Daggers in the area that the Stirge and Fighter both occupy.

You fill the air with spinning daggers in a cube 5 feet on each side, centered on a point you choose within range. A creature takes 4d4 slashing damage when it enters the spell’s area for the first time on a turn or starts its turn there.

Therefore when I finish my turn, the Stirge's turn starts and immediately takes 4d4 damage killing it immediately as it has a 2 HP. Then I would like to drop my concentration immediately before the Fighter's turn so that he wouldn't take the subsequent damage.

If a spell must be maintained with concentration, that fact appears in its Duration entry, and the spell specifies how long you can concentrate on it. You can end concentration at any time (no action required).

According to the emphasised sentence in the above quote, I can drop concentration at any time, which I assume to mean even out of my turn. However the Fighter was encased in the Cloud of Daggers, so how does the spells description interact with the concentration rule here?

My understanding from a purely RAW interpretation is that the Fighter takes no damage as I can drop the concentration before the start of his turn. Can anyone confirm if I'm missing any other rules that might effect this sequence of events, as my common sense tells me the Fighter probably should take some damage by being in the cloud for a brief period of time?