Medication - How long does 15 mg of phenobarbital take till half life?

Phenobarbital has a very long half-life, and this can vary greatly among individuals. According to the prescribing information for phenobarbital the half-life is as follows:

• Newborn babies (<48 hours old), Infants, and Children: Average half-life is 110 hours (range is 60 to 180 hours)
• Adults: Average half-life is 79 hours (range is 53 to 118 hours).

The half-life of a drug is an estimate of the time it takes for the concentration or amount in the body of that drug to be reduced by exactly one half (50%). After four to five half-lives, 97% of a drug has cleared from the body, and the drug is no longer considered to be having an effect, although, for most drugs, actual noticeable effects would have worn off well before then. The symbol for half-life is T½.

For example, if 15mg of phenobarbital is taken, using the average half-life of 79 hours for adults, the following is estimated:

• 79 hours (3.3 days) after administration, 7.5mg remains
• 158 hours (6.6 days) after administration, 3.75mg remains
• 237 hours (9.9 days) after administration, 1.88mg remains
• 316 hours (13.2 days or almost 2 weeks) after administration, 0.94mg remains
• 395 hours (16.5 days) after administration, 0.47mg remains.

In theory, we can see that after 16.5 days or over two weeks, almost 97% of this drug is expected to have been eliminated. However, this does not mean that it won’t be detectable by a drug test, as this depends on how specific and sensitive the drug test is.

In reality, the half-life of phenobarbital varies from person to person, and even sometimes within the same person. The half life of phenobarbital can also be affected by dosage and a number of other factors including:

• Age
• History of previous drug use
• Pre-existing conditions
• Presence of other drugs that compete for binding sites or interact in other ways
• Race/ethnicity (this can influence the metabolism of a drug).

See our article Drug Half-life Explained for more general information about half-lives.