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→Physiological Variables
===Physiological Variables===
===Physiological Variables===
* '''pH at site of absorption'''
Many drugs are either a '''weak base''' or a '''weak acid''' and so will exist in both an unionised and ionised form in the same solution. The ratio of the two forms is dependent upon the pH of the location of the drug.
The ionisation of a weak acid is
AH<sup>+</sup> =<sup>'''Ka'''</sup> A<sup>-</sup> + H<sup>-</sup>