# Absorption rate constant

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The **absorption rate constant** ***Ka*** is a value used in [pharmacokinetics](/source/Pharmacokinetics) to describe the rate at which a [drug](/source/Drug) enters into the system. It is expressed in units of time−1.[1] The Ka is related to the [absorption half-life](/source/Absorption_half-life) (t1/2a) per the following equation: Ka = ln(2) / t1/2a.[1]

Ka values can typically only be found in research articles.[2] This is in contrast to parameters like [bioavailability](/source/Bioavailability) and [elimination half-life](/source/Elimination_half-life), which can often be found in drug and pharmacology handbooks.[2]

## References

1. John E. Murphy (1 July 2016). [*Clinical Pharmacokinetics*](https://books.google.com/books?id=0P07DwAAQBAJ&pg=PT26). ASHP. pp. 26–. ISBN 978-1-58528-538-9.

1. Miles Hacker; William S. Messer; Kenneth A. Bachmann (19 June 2009). [*Pharmacology: Principles and Practice*](https://books.google.com/books?id=5YDMmjWXe-AC&pg=PA257). Academic Press. pp. 257–. ISBN 978-0-08-091922-5.

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