Learn German: Sie ist früher nicht allergisch gewesen.

When you want to negate a specific adjective or quality, nicht goes directly in front of the word it negates—in this case, allergisch. Also, in the spoken past tense (Perfekt), nicht cannot go at the very end of the sentence; it must come before the past participle (gewesen).

Why does the time word früher come before nicht?

Time words like früher usually come as early as possible in a German sentence, right after the main verb. If you swapped them and said nicht früher, you would be specifically negating the time, meaning "not earlier" (but perhaps later), rather than stating she wasn't allergic.

Why use ist ... gewesen instead of the simpler war?

Both Sie war früher nicht allergisch (simple past) and Sie ist früher nicht allergisch gewesen (present perfect) are grammatically correct. While native speakers frequently use the shorter war for the verb sein, the ist ... gewesen form is still used in spoken German, especially when emphasizing a completed state.

Could I use vorher instead of früher in this sentence?