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No, not necessarily, but it can cause a minor issue. iPhones use a Lithium-Ion battery for its power, using while it's charging won't necessarily ruin the battery, but if you're using the phone while the battery is in the charging process, it can cause the phone to think it has more power than it actually has.
Thought it will show 100% charge within 1 hour, you should leave it on for full 8 hours with the phone off.. do not use it while charging..once the 8 hour first charge is complete, then your battery is good to go.. it will give you good battery life.