Routes of drug administration

Wikis > Pharmacology > Pharmacokinetics > Absorption > Routes of drug administration

Routes of drug administration:
The route of administration affects the rate of onset of action and the magnitude of the therapeutic response.
i) Enteral (‘enteral’= the digestive tract)
1) Oral
• most common route, due to convenience and safety – but can be somewhat unreliable due to GI tract changes from emotion, food and physical activity
• slightly acidic pH of mouth limits absorption there (better absorption of those drugs that dissolve in saliva
• easy, inexpensive and convenient route of administration
• barriers to absorption are epithelial cells lining GI tract and capillary walls
• absorption is affected by rate of stomach emptying, food ingestion, solubility of drug, ph, interactions with food or other drugs, special coatings on drug and presence of GI disease
• tablets (drug mixed with base, binder and fillers), capsule (gelation surrounds drug), mixture (drug dissolved or diffused in water) or emulsion (mixture of two liquids)
• disadvantages – absorption is very variable; can inactivate some drugs; unconscious or unco-operative patient; can irritate GI tract
2) Sublingual (under tongue) or Buccal (across membranes of the mouth)
• both allow rapid absorption
• not affected by first pass metabolism
3) Rectal
• good blood supply in rectal mucosa
• suppositories – melt at body temperature  release drug
• used when cannot take medicines orally (eg vomiting)
• by passes liver (decreased first pass metabolism)
• not a convenient method

ii) Parenteral (‘par’= besides)
Direct administration of drug into tissues.
1) Intravenous (IV)
• into systemic circulation  instant and rapid effect
• more precise control over drug levels in the blood
• either bolus (single dose) or infusion over time
• no barriers to absorption  complete bioavailability
• some drugs can only be administrated this way due to potential to be irritant or they are highly chemically reactive
• technically more difficult than other routes; more expensive; more inconvenient; potential to be more dangerous (eg overdose); potential for infection at injection site; has risk of embolism
• preferred method if drugs have short half life or when dosage has to be carefully monitored

2) Intramuscular (im)
• absorption from ingestion site is variable – depends on blood flow, activity levels and rubbing of tissues
• only barrier to absorption is capillary wall – absorption will depend in blood flow at site of injection and water solubility of drug (rapidly absorbed if highly water soluble).
• good method for poorly soluble drugs
• useful for drugs that can be absorbed gradually over time – ‘depot preparations’
• eg into gluteal muscle
• technically easier than IV, but IM can be painful/uncomfortable

3) Subcutaneous (sc)
• given by injection into the connective tissue or fat below dermis
• eg insulin
• only barrier to absorption is capillary wall
• lower absorption rates than im

4) Inhalation:
• inhaled into lung – especially useful for drugs that are broken down in GI tract or when need direct effect on respiratory tract
• eg asthma drugs; insulin is being trialed; inhalation general anaesthetics

5) Intrathecal:
• used when need to inject CNS and bypass blood/brain barrier
• injected directly into the spinal subarachnoid space

6) Epidural:
• injected directly via a small catheter into the spinal cord

iii) Other:
1) Topical
• applied to skin or mucous membrane where they are needed
• action is specific to site of application
• eg lotions, creams

2) Drops
• eg eye drops

3) Sprays (respiratory route)
• eg nasal sprays; bronchodilators

4) Transdermal
• drugs absorbed via skin
• bypasses first-pass hepatic inactivation
• used for skin diseases or systemic conditions if absorption is sufficient
• plastic patch holds drug in place
• absorption is variable
• eg oestrogens for hormone replacement therapy

Most routes require that the drug be absorbed across a lipid membrane before entering the systemic circulation. Absorption can happen by a number of processes:

i) Aqueous diffusion:
• if drug is small enough, it can pass through pores
• very few drugs cross membranes this way – only smallest of compounds (molecular weight <200)ii) Passive diffusion/transport: • if lipid soluble, smaller molecule and concentration gradient exists across the membrane  diffusion will continue until concentrations are equal on both sides of membrane • requires no cellular energyiii) Carrier mediated diffusion: • if not lipid soluble, the drug can become lipid soluble by combining with carrier  diffuses across membrane  dissociates once crossed membrane.iv) Active transport: • requires cellular energy to move drug from an area of low concentration to an area of high concentration. • eg levodopa

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