Although it is sold over-the-counter, melatonin isn't approved by the Food and Drug Administration and researchers disagree on the optimal dose.

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"It's going to be a few years until we really know whether or not people need another booster dose, and what is the optimal timing for that booster dose" for Tdap, because it is so new, Martin said.

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Engineers had to calculate the optimal air flow into the chamber to ensure that patients would breathe a full dose each time.

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Calabrese says that many poisons follow, instead, a U-shaped curve: The death rate goes down as the dose climbs from zero to a certain small amount, what you could call the optimal exposure.

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