iPhone battery health dropped quickly in a few months

It's usually normal. Battery health will decline on average about 1% per month.

However... the drop is not usually linear. It will hold steady for a while, then drop several points at once or in quick succession before stabilizing again for a while.

When it reaches 80%, it's time to get the battery replaced.

The rate of battery decline varies with different models of the iPhone and of course with how you use the phone daily.

Tap Settings > Battery > Battery Health & Charging. What is the current Maximum Capacity?

Do you see a setting for Charge Cycles? If yes, how many Charge Cycles have occurred at this point?