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→Formulation
===Formulation===
===Formulation===
For a drug to be absorbed it must be in an aqueous solution. Thus the rate of absorbtion of an injectable formulation is influenced by it's aqueous solubility. As the solubility decreases the rate of absorbtion and thus its plasma concentration is lower.
When considering formulations of drugs it is important to look at their Cmax and Tmax values.
'''Cmax''' = The maximum concentration that the drug will reach in that formulation in the body system.
'''Tmax''' = The time it takes to recah the Cmax.
====Oral Formulations====
Usually 75% of a drug is absorbed in 2-3 hours when given orally. The following factors affect their absorption:
* Tablet structure and size - this can be altered to change Tmax and Cmax
* Stomach enzymes - can breakdown the drug
* Stomach acidity - some drugs are unstable in such an acidic environment, others will be ion trapped
* Gastric motility - with increased motility there is less time for the drug to absorbed
* Damage to the epithelial barrier - either due to drugs or disease will result in reduced absorbtion
* Stability of the drug in the gastrointestinal tract
* Presence of food - it can delay drug absorption without changing the amount absorbed, it can delay the absorption and decrease the amount absorbed or it can increase the rate and amount absorbed.
* First Pass Metabolism - a drug can be extensively metabolised before reaching the systemic circulation. This can occur in the gastro-intestinal tract wall, in the portal circulation and in the liver.
====Bioavailability====
This is the fraction of the administered drug that actually reaches the systemic circulation in its active form. It is expressed as a percentage of the total amount administered and is calculated using plasmas drug concentrations.