Ways of taking medication
Now that we have seen what the body does to drugs, we can focus on the way each formulation (the form of a drug) can affect absorption, distribution, bioavailability and excretion.
The various forms of taking medication:
Tablets
Suppositories and pessaries
Sublingual
Transdermal
Inhalants and nasal sprays
Eye drops
Intravenous (IV)
Intrathecal
Intramuscular (IM)
Subcutaneous (SC)
Tablets
Most medications are available as tablets - taken by mouth and swallowed. This is often the easiest way to administer a drug. This formulation works well for acidic drugs (as the inside of the stomach is acidic), or fat-soluble drugs (which are absorbed into your cells quickly). Drugs that are proteins, such as insulin or the interferon beta class of drugs, are broken down in the stomach in the same way as the protein in food, meaning that oral administration is not very effective.
Suppositories and pessaries
Each drug is designed both for a specific illness and a specific route of administration. Suppositories, placed in the rectum, and pessaries, placed in the vagina, can be unpleasant but are useful if a ‘local effect’ is desired – in other words, if the drug is being used to treat the colon, vagina or womb. Also, suppositories are useful if someone is nauseous or vomiting, or cannot take medication by mouth.
Sublingual
Drugs given under the tongue (i.e. sublingually) are useful when both a rapid response and self-administration are needed. The drug goes quickly into the bloodstream via the delicate cells lining the mouth, therefore avoiding the ‘first-pass effect’ of tablets that are swallowed. One example is glyceryl trinitrate, which is given sublingually for angina.
Transdermal
Medication delivered transdermally enters the body via the skin. It is given either as a cream or as a patch. Creams are most often used when a local effect on the skin is desired (e.g. for an insect bite or rash).
Patches contain a reservoir of drug, which is released slowly and evenly over a period of time. The drug passes through the skin and into the bloodstream for onward transport through the body to its target tissue.
Transdermal delivery of nicotine in patches reduces the withdrawal symptoms that accompany stopping smoking. Patches are also used to deliver hormone replacement therapy for menopause, glyceryl trinitrate for angina and hyoscine for seasickness.
Inhalants and nasal sprays
Inhalation is used for volatile and gaseous anaesthetics, and drugs that affect the lungs (e.g. for asthma). Inhaled drugs pass quickly to the bloodstream, avoiding the ‘first-pass effect’. The lungs contain a vast network of tiny air sacs (alveoli) which provides a large surface area through which the drug can travel from the inhaled air to the bloodstream.
In addition to commonly used decongestants, nasal sprays are sometimes used for very small protein drugs (called peptides) to avoid the need for frequent injections. Examples include gonadotrophin-releasing hormone (for infertility), and antidiuretic hormone (ADH - to promote water retention). You may know ADH as desmopressin, which may help to prevent urination during the night in MS. Antihistamines are also sometimes given through nasal sprays to treat colds, so that the drug can be delivered directly to the source of congestion.
Eye drops
Eye drops are almost exclusively used to deliver a drug directly to the eye (e.g. to treat glaucoma or an eye infection). To be effective, however, the drugs in eyedrops need to be fat soluble. Some absorption into the bloodstream does occur.
Injectables: intravenous (IV)
Intravenous administration involves injecting a liquid containing the drug directly into veins. It is the quickest and most direct way of delivering a drug, and avoids the ‘first-pass effect’. The bolus of drug goes first to the heart and then to the general circulation. Some drugs are given all at once; others are delivered over a certain period of time (an infusion). You may have had intravenous injections of steroids such as methylprednisolone.
Injectables: intrathecal
Intrathecal administration involves placing the needle into the area around the spinal cord to deliver a drug directly to the central nervous system. Severe spasticity in MS is sometimes treated with baclofen, which can be administered intrathecally. Spinal anaesthesics (e.g. when used in labour) are also administered intrathecally.
Injectables: intramuscular (IM)
Some drugs are injected directly into muscle. Since muscle has a rich blood supply, it provides a faster route to the rest of the body than oral administration. Massaging the muscle after injection can increase blood flow and therefore distribution to the rest of the body.
If slow administration is needed, drugs given intramuscularly can be provided in a slow-release or depot form.
IM injections can be quite painful or uncomfortable because they penetrate deep into the muscle layers, which are richly supplied with nerves. Bruising may occur if a blood vessel is punctured during an injection. IM drugs do not have to be given as often as IV or subcutaneous (see below) injections, but they should be given by a healthcare professional or under the supervision and guidance of a healthcare professional because they have the skills needed to give such a deep injection: they know how to avoid hitting a bone or nerve.
Injectables: subcutaneous (SC)
Subcutaneous injections deliver the drug to just below the fatty layer underneath the skin. They are not very painful because there are very few blood vessels or nerve endings in the subcutaneous area. The drug is usually absorbed more slowly (and therefore steadily) than those given intramuscularly. Depot drugs can be used to control the rate of absorption. There is a limit to the amount of drug that can be delivered by SC injections (large volumes are not feasible). Massaging the injection site helps with absorption, as with IM injections. Localised injection site reactions can occur but these can be managed by rotating the injection sites.
Although people who need to inject themselves subcutaneously require initial instruction from a healthcare professional, they can learn to perform the injections on their own, providing flexibility and independence. Many people regularly self-inject drugs subcutaneously.
The interferon beta class of drugs are proteins and so cannot be taken orally because they would be broken down in the stomach. For this reason they are injected either SC or IM. Interferon beta-1b e.g. is given by subcutaneous
