Last week, in a meeting with senior staff from the FDA responsible for labeling, we were dumbfounded. We learned that these pharmacologists did not regard it as important for clinicians to know the half-life of posaconazole in persons with obesity. In fact, they told us “it could potentially be confusing.”

On the spot this was troubling. Simply because it goes against everything we intuitively know as a pharmacist.

Yes, It’s Important

But rather than rely on a gut reaction, we decided to consult authoritative sources. This is what we found.

The elimination half-life of a drug is one of the most important pharmacokinetic parameters because it directly affects dosing frequency, drug accumulation, time to reach steady state, the duration of effect, and the time-course of washout after discontinuation. Clinicians and clinical pharmacists rely on it to:

1. Determine dosing intervals: A drug with a short half-life may need to be dosed several times a day, while one with a long half-life can be given less often.

2. Predict time to attain steady state: It generally takes about 4–5 half-lives for a drug to reach steady-state concentration with repeated dosing.

3. Guide withdrawal and washout periods: Half-life helps predict how long a drug (and its effects) will persist after discontinuation, which is important when switching therapies or avoiding interactions.

4. Understand accumulation and toxicity risks: Drugs with long half-lives may build up if not dosed carefully, increasing the risk of adverse effects.

Well-Established Importance

The importance of understanding a drug’s half-life is hard to dispute. Perhaps the most authoritative reference for understanding pharmacokinetics and the concept of elimination half-lives is Goodman & Gilman’s The Pharmacological Basis of Therapeutics (14th edition, 2023). In Chapter 2 (Pharmacokinetics: The Dynamics of Drug Absorption, Distribution, and Elimination), the authors explain that half-life is one of “the four most important parameters governing drug distribution.”

For StatPearls in the National Library of Medicine, Jericho Hallare and Valerie Gerriets explain that elimination half-life is one of the oldest and most fundamentally important pharmacokinetic parameters that clinical professionals must understand:

“Examiners of the United States Medical Licensing Examination regularly test and assess medical students and licensed clinicians to ensure they understand it.”

Another authoritative source is FDA guidance for industry on pharmacology in drug labeling (2016), which requires inclusion of half-life in prescribing information because understanding it is essential for dosing and safety. Indeed, elimination half-life is important information that belongs in drug labels because FDA says so.

Confusing?

So the elimination half-life of a drug is an important parameter that clinicians utilize every day. To suggest that FDA should withhold the information on a drug with a prolonged half life in obesity strikes us as both absurd and condescending. This is information that clinicians tell us they need.

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Adam and Eve and the Forbidden Fruit of Knowledge, painting by Marc Chagall / WikiArt