If need be + ...

[Jadoon 84]

Is this sentence natural?

If need be, take another dose. If yes then please convert this sentence into past and future tense.

Thank you.

[Grumpy]

Is this sentence natural?

If need be, take another dose. If yes, then please convert this sentence into past and future tenses.

Thank you.

There are two, apparently unrelated, sentences in your post.
Both are correct.