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XIII Formulation factors


=> To understand the formulation factors which affect the oral absorption of drug products

The role of the drug formulation in the delivery of drug to the site of action should not be ignored. With any drug it is possible to alter its bioavailability considerably by formulation modification. With some drugs an even larger variation between a good formulation and a bad formulation has been observed. Since a drug must be in solution to be absorbed from the G-I tract, you may expect the bioavailability of a drug to decrease in the order solution > suspension > capsule > tablet > coated tablet. This order may not always be followed but it is a useful guide. One example is the results for pentobarbital. Here the order was found to be aqueous solution > aqueous suspension = capsule > tablet of free acid form. This chapter will briefly discuss each of these formulation types particularly in regard to the relative bioavailability.


Drugs are commonly given in solution in cough/cold remedies and in medication for the young and elderly. In most cases absorption from an oral solution is rapid and complete, compared with administration in any other oral dosage form. The rate limiting step is often the rate of gastric emptying.

When an acidic drug is given in the form of a salt, it may precipitate in the stomach. However, this precipitate is usually finely divided and is readily redissolved and thus causes no absorption problems. There is the possibility with a poorly water soluble drug such as phenytoin that a well formulation suspension, of finely divided powder, may have a better bioavailability.

Some drugs which are poorly soluble in water may be dissolved in mixed water/alcohol or glycerol solvents. This is particularly useful for compounds with tight crystal structure, higher melting points that are not ionic. The crystal structure is broken by solution in the mixed solvent. An oily emulsion or soft gelatin capsules have been used for some compounds to produce improved bioavailability.

This page was last modified: 12 February 2001

Copyright 2001 David W.A. Bourne

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