GASTROINTESTINAL DRUG ABSORPTION
OF DOSAGE FORMS – SOLUTION
(SYRUP, ELIXIR) AND SUSPENSION

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Nature and type of dosage form

▪ Apart from the proper selection of drug, clinical success
often depends to a great extent on the proper selection
of dosage form of that drug

▪ For a given drugs more difference could be observed in
the oral bioavailability of a drug depending upon the
nature and type of dosage form.

▪ Such difference is due to relative rate at which particular
dosage form releases the drug to the biological fluids and
the membrane.

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▪ The relative rate at which a drug from a
dosage form is presented to the body
depends upon the complexity of dosage
form.

▪ The more complex a dosage form, greater
the number of rate limiting steps and greater
the potential for bioavailability problems.

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▪ As a general rule the bioavailability of a drug
from various dosage forms decreases in the
following order;

Solutions > emulsion > suspensions > capsules>
tablets> coated tablets> enteric coated
tablets > sustained release products

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▪ Thus absorption of a drug from solution is
fast with least potential for bioavailability
problems whereas absorption from sustained
release product is slowest with greatest
bioavailability risk.

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Solutions

▪ A drug in a solution (syrup, elixir etc) is most
rapidly absorbed since the major rate limiting
step, drug dissolution, is absent.

▪ Factors that influence bioavailability of a
drug from solution dosage form include- the
nature of solvent ( aqueous, water miscible
etc), viscosity, surfactants, solubilisers,
stabilizers,etc.

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▪ Quite often, dilution of a drug in solution with
GI fluids results in precipitation of drugs as
fine particles which generally dissolve rapidly.

▪ Factors that limit the formulation of a drug in
solution form include stability, solubility,
taste, cost of the product etc.

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▪ Complexation : formation of a complex between
the drug and an excipient included in the
formulation to increase the aqueous stability,
chemical stability or viscosity of the dosage form

▪ Solubilization : incorporation of the drug into
micelles to increase the aqueous solubility of the
drug.

▪ Viscosity : viscosity of the solution, particulary if
the viscosity enhancing agent has been included.

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Suspensions

▪ The major limiting step in the absorption of a
drug from suspension dosage form is drug
dissolution which is generally rapid due to the
surface area of the particles.

▪ Important factors in the bioavailability of a
drug from suspensions include particle size,
polymorphism, wetting agents, viscosity of
the medium, suspending agents etc.

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▪ Particle size and effective surface area of the
dispersed : small particle size and thus a large total
surface area facilitates dissolution and hence
absorption of drugs

▪ Crystal form of the drug
Some of the drug change their crystalline structure in a

suspension, which may not be desirable.
e.g. sulphathiazole can change to its other polymorphic

forms and this can be overcome by the addition of
PVP

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▪ Complexation: formation of a non absorbable
complex between the drug and the other ingredients
of the formulation leads to poor bioavailability

▪ Inclusion of surfactant as a wetting or flocculating
agent

▪ Viscosity of the suspension
In case of suspension an increase in viscosity could lead

to a decrease in the rate of dissolution of the drug in
the GIT

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▪ Inclusion of colorants
certain dyes like erythrosine and brilliant

blue when added to suspension of diethyl
stilbesterol, phenobarbitone reduce the
dissolution rate by the extent of 15%.

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Reference

▪ Biopharmaceutics and pharmacokinetics by D
M Brahmankar and Sunil B. jaiswal

▪ Text book of biopharmaceutics and
pharmacokinetics by Dr. Shobha Rani R.
Hiremath

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Thank you

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