Lipid water solubility coefficientis the ratio of dissolution of drug in lipid as compared to water. Greater the lipid water solubility coefficient, more is the lipid solubility of the drug and greater is the absorption. Less the coefficient, less is the lipid solubility and less is the absorption.

Water film exists on the membranes so part of the drugs must be water soluble to cross this water film

Smaller the molecular size of the drug, rapid is the absorption.

Particle may be composed either of a single molecule or more than hundred molecules

Different drugs are either acidic or basic and are present in ionized or unionized form, which is given by their pKa values.

Drugs may exist as solids, liquids or gases. Gases are rapidly absorbed than the liquids, while the liquids are rapidly absorbed than the solids. Thus the drugs in syrup or suspension form are rapidly absorbed than the tablets or capsules.

Chemical nature is responsible for the selection of the route of administration of drug

Dosage forms affect the rate and extent of absorption. A drug can be given in the form of tablets, capsules or transdermal packets. Injections may be aqueous or oily. This changes the rate of absorption.

When the drugs are formed, apart from the active form some inert substances are included. These are the diluents, excipients and the binders. Normally they are inert, but if they interact, they can change the bioavailability.

According to Fick’s law, higher the concentration more flux occurs across the membrane. The rate is less affected than the extent of absorption.

Area of absorptive surface affects oral as well as other routes. Most of the drugs are given orally because of the large area of absorptive surface, so that greater absorption occurs. .

More the vascularity, more is the rate and extent of absorption and vice versa. In shock, blood supply to the GIT is less so the oral route of drug administration is affected. The blood flow to the peripheries is decreased, so absorption in those areas is diminished as well. Therefore, intravenous route is preferred in case of shock

The distribution of a weak electrolyte across a membrane will be determined by its pKa and the pH. Many drugs are weak electrolytes and tend to ionize according to environmental pH:

Foods or drugs may interact with the drugs to alter their rate of absorption. Especially for the drugs given orally, food can increase or decrease the absorption.

GI mobility must be optimal for absorption of oral drugs. It should be neither increased nor decreased which may affect the rate or extent of absorption. Different diseases or drugs may alter the mobility

Flattening and edema of mucosa decreaes absorption. Dysfunctional breach in the skin affects the absorption of topical drugs.

a. Diarrhea Decreases absorption because of decreased contact time. b. Malabsorptive syndrome Decreases absorption c. Achlorhydria Acidic medium for acidic drugs is affected. d. Cirrhosis Cirrhosis affects portal circulation. Thus affecting metabolism of drugs. e. Emphysema Emphysema affects the absorption of volatile gases through the pulmonary route. f. Lipodystrophy Lipodystrophy decreases absorption. In diabetics, insulin might lose its affect.