The process by which a drug enters the blood circulation from the site of administration is called drug absorption. Many factors can affect the absorption of drugs, such as the physical and chemical properties of the drug, the dosage form, the formulation and the route of administration. The effect of the route of administration on absorption is of the utmost importance. Different routes of administration can directly affect the extent and speed of drug absorption.
Oral tablets and capsules are administered orally, and it is the most commonly used route of administration. Its advantages are convenient, economical, and safe for most patients and drugs. Its shortcomings are slow absorption and irregularity, and it is easily affected by gastrointestinal contents, gastrointestinal motility. Additionally, oral medications must first pass through the liver. If the liver has a strong metabolic capacity or a large amount of excretion from the bile, the amount of effective drug that enters the systemic blood circulation is significantly reduced. It is not suitable for coma, vomiting, convulsions, and acute patients. In addition, drugs that are easily destroyed by digestive enzymes are not suitable for oral administration.
Compared with oral route, injection administration is much more complete, rapid, effective, and accurate, but less economical, convenient, and safer.
- 1. Intravenous injection: the drug directly enters the blood, without absorption process, and quickly exerts the efficacy, especially suitable for critical patients. But intravenous administration has certain risks, and the dosage and speed of administration should be strictly controlled. Oils and suspensions should not be administered intravenously.
- 2. Intramuscular injection: muscle tissue is rich in blood vessels, so this route makes the drug absorption faster and more complete, and the sensory nerve fibers are less, so the pain is lighter. Intramuscular suspension or oil has slow absorption and long-lasting effect.
- 3. Subcutaneous injection: the drug absorption is slow and uniform, but faster than oral administration, and the drug effect lasts for a long time. Irritant drugs and oils may cause pain, inflammation and induration at the injection site, and should not be injected subcutaneously.
Gas, volatile liquid and aerosol medicines are often administered in the respiratory tract. The drugs are absorbed through the alveoli, rapidly entering the blood to produce a therapeutic effect or function locally in the respiratory tract. The effect of this route is rapid and short-lived. The disadvantage of this method is that it is irritating to the respiratory tract.
Drugs with higher fat solubility and smaller dosage can be administered by sublingual administration. The drug is absorbed by the sublingual mucosa but has a small absorption area.
It can avoid the destruction of drugs by the gastrointestinal fluid and the stimulation of the gastrointestinal tract by the drug, and largely avoids the elimination of the first pass. However, it makes the absorption area of the drug small and the absorption is irregular, and is suitable for those who have strong gastrointestinal irritation and difficulty in taking medicine for children.
The drug is applied to the surface of the skin or mucous membranes. Most drugs exert local effects, and some drugs can exert systemic effects.