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From a case report:

In addition, the authors started 5-hydroxytryptophan titrated to 400 mg orally twice daily with concomittant carbidopa 37.5 mg orally four times a day, and he responded with remission of suicidal ideation and significant improvement in depression and function.

Does "400 mg orally twice daily" translate into 800 mg of the drug per day, or does it mean "a total of 400 mg/day was divided into two daily doses"?

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Yes, and that the recommended interval for dosage is twice daily.

The convention for BID (twice daily) dosing is to write "400 mg twice daily" to indicate the dosage at each administration. Therefore it would be 400 mg + 400 mg = 800 mg total daily.

With some medications, the dosage is listed as daily total, divided into BID or TID dosing. For example, "1200 mg daily divided into BID or TID dosing" would be 600 mg twice daily, or 400 mg three times daily.

This entry for amoxicillin shows how it can be written both ways, though I have seen better wording in other references (like UpToDate or Epocrates but both require registration or paywall)

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    Please let me know if this is different outside of the US or if you have another reference that better demonstrates this.
    – DoctorWhom
    Mar 18, 2019 at 10:14
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    Can confirm this is the same in the UK and Ireland. Occasionally dosage advice (e.g. in the British National Formulary) may say (for example) "200mg daily in divided doses", which means the number of doses per day is left to the discretion of the prescriber. However, patient-facing dosing instructions are always the dosage at each administration.
    – Chris
    Mar 18, 2019 at 15:45

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