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A crude way of looking at how we get medicines into the body is this – “if there’s a hole then we’ll put medicines in it and if there is not a hole, then we’ll go and make one”! The intake of medicines is achieved either via absorption or direct injection. With absorption the product might be (for example) put on the eye or skin, ingested and then absorbed in the intestine or breathed in and absorbed in the nose or lungs. Most medicines are taken this way. With direct injection the body is injected directly into the bloodstream or other body fluid. There are a wide range of methods of administration of medicines. These are as follows:

  • Oral
  • Injection
  • Inhalation
  • Topical
  • Rectal
  • Transdermal
  • Sublingual
  • Ocular
  • Aural
  • Vaginal

I will now run through these in a little more detail.


The most common method of getting drugs into the body. Generally seen as convenient by the patient. The drug formulation needs to protect the active ingredient through the acids of the stomach. Once absorbed the drug passes through the liver where it will be metabolised.

Intravenous injections

These drugs are injected straight into the bloodstream and have a rapid distribution and effect. They also avoid the initial metabolism in the liver. They can be used for small administration of product via injection or for longer term dosage via intravenous bags. Due to the fact that these products bypass the body’s natural defence mechanisms they need to be sterile (free from micro-organisms).

Other specialised methods of injection also exist. These include:

  • Intra-muscular injections (into the muscle)
  • Subcutanious injections (into the skin layers)
  • Intra-cisternal injections (into the lymph ducts)
  • Intra-osseous injections (into the bone marrow)
  • Intra-articular injections (into a joint)
  • Intra-peritoneal injections (into the body cavity around the gut and intestines)
  • Intra-arterial (into an artery) – normally you are injected into a vein
  • Intra-cardiac (into the heart)
  • Intra-cranial (into the skull)


These products are breathed in and are absorbed into the blood stream via the lungs. Examples include anaesthetic gasses, asthma inhalers and nebulisers. They do not need to be sterile, but aqueous inhalation products (nebulisers) often made and tested as if they were sterile. The USA required aqueous inhalation products to be sterile.


These products are applied directly onto the outer body surfaces. They include creams and ointments. The drug is not well absorbed through the skin and as such these products are used to treat the condition locally.


Here the product is administered into the anus (bum, bottom, anus, arse (whatever you want to call it)) in the form of a suppository. This is actually a very good route of drug administration due to the high qualities of blood vessels around the anus. Treat customer complaints from individuals who complain that a suppository doesn’t taste very nice carefully! The patient has put the product in the wrong hole!


Used as patches on the skin for long term dosing. Good if a steady amount of the drug is required over a period of time (the user can, for example, forget to take a tablet). Commonly used for smoking cessation products, hormone replacement and pain relief during late stages of cancer.


Here the product is absorbed under the tongue. The dry product is taken into the mouth where it dissolves. It is absorbed rapidly with quick onset of action. This method also avoids initial metabolism in the liver.


Here a sterile product is added to the eye. Eye care products must be sterile because the lens of the eye cannot repair itself as it has no blood supply.


Here products are added to the ear, usually to treat conditions of the ear such as ear infections and dealing with excessive ear wax.


These types of product are called pessaries and works as a similar way to a suppository. Their usage is restricted to about half of the world’s population – I can’t think why!

This topic and many others is covered in our on-line course on Medicinal Chemistry.
I hope this post is of interest. Dominic

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