11. Drug formulations. Prescription writing

Page created on October 11, 2018. Last updated on January 7, 2022 at 21:55

Drug formulations

Drug formulations are different ways to administer drugs, like intravenous, by tablet, by eyedrops, etc. They have different advantages and disadvantages.

Oral medication

All oral medications are taken by mouth and most are also swallowed.

Their advantages:

  • Convenient
  • Economical

Their disadvantages

  • Nausea, vomiting
  • Irritation of gastric mucosa
  • Unpleasant taste or smell
  • Drugs must be absorbed via GIT -> needs time, can vary between patients
Liquid oral medications
Formulation Description Advantage Disadvantage
Solution Drug is dissolved in water Nothing special Nothing special
Syrup Drug is in a homogenous, sugar-based solution Sweet taste Nothing special
Elixir Drug is dissolved in alcohol and sweetened Some drugs are only soluble in alcohol Alcohol cannot be given to every patient
Suspension Drug is suspended in water Some drugs aren’t soluble and must be given in suspension Container must be shaken
Oil or emulsion Drug is inside an oil Nothing special “Shitty” taste
Solid oral medication

Tablets are an important type of solid oral medication. They’re made by compressing the drug in powder form into solid tablets. Tablets don’t just contain the drug. If they did, certain tablets would be so small that they would be very difficult to handle. Tablets therefore have other components as well, called excipients. These excipients can be binders that bind ingredients together, disintegrators that allow the tablet to absorb fluid and therefore disintegrate in the GIT. Lubricants, sugars, menthol and dyes may also be added.

Tablets have different shapes and colours so that they’re easier to handle and, more importantly, identify.

Different types of tablets

The advantages of tablets are:

  • Convenient
  • Economical
  • Easy to give accurate dose
  • Compact
  • Easy to administer
  • Portable

Certain special types of tablets exist:

Type of tablet Description Advantages
Sugar-coated (dragees) Coated with sugar to give better taste Better taste
Enteric-coated (EC) Will only be dissolved in duodenum Prevents drug from being broken down before duodenum
Sustained-release Drug released slowly and constantly in GIT Blood level of drug is stabile over longer period
Chewable Must be chewed before swallowing Easier to swallow
Oral osmotic therapeutic system (OROS) Inside GIT water will enter the tablet and cause the tablet to disintegrate Drug is only released in GIT
Sublingual Tablet is kept under tongue Drug bypasses the liver, prevents first pass effect
Buccal Drug is kept in the inside of the cheek Drug bypasses the liver, prevents first pass effect
Hard capsules

Capsules are another type of solid oral medication. They are small gelatine cases that enclose the drug in powder, granule or liquid form. The gelatine case of the capsule dissolves in the stomach, which will release the drug contents.

Two types of capsules exist. The hard type has two pieces that contain drug as the solid. The soft type has one piece and contains the drug in liquid form.

Soft capsules
Transdermal medication

Drugs can be given through the skin by adding patches that contain lipophilic drugs to the skin. This causes the drugs to be absorbed directly into the blood stream, which circumvents the liver and prevents the first-pass effect.

This is used for oestrogen, nicotine, melatonin, organic nitrates and analgesic drugs.

Vaginal medication

Different formulations like creams, solutions and ointments can be used intravaginally to treat local infection or irritation, as a contraceptive or as a hormone replacement therapy in menopause.

Rectal medication

Drugs can be given rectally when oral administration is impossible or contraindicated, like when the drug can irritate the gastric mucosa, the patient is unable or unwilling to swallow the tablet, if some local problem in the rectum must be treated or when we want to avoid the liver and the first-pass effect.

Looks like a rocket

These drugs have a conical shape and can be covered in lubricant to east the administration. They can be given either for a local effect or a systemic effect.

Ophthalmic medication

Medications can be given directly onto the eye when we want to diagnose, treat or prevent eye diseases, like infections or glaucoma. Some drugs are given ophthalmically to induce pupil dilation so that the eye examination will become easier.

Formulations used on the eye include solutions, suspensions and ointments. It’s essential that whatever formulations is used must be sterile.

Otic and nasal medication

Drugs in liquid form can be instilled into the external auditory canal to treat local inflammations or infections.

Nasal medication is very common. They’re often given as drops or a spray and is mostly used for local effects to decongest the nose. Peptide hormones are given nasally to achieve systemic effects, like vasopressin and oxytocin.

Topical cutaneous medication

Many different types exist, like

  • Emollients
  • Lotions
  • Ointments
  • Creams
  • Solutions
  • Soaps
  • Shampoos

They’re used to treat the local skin or mucous membrane for

  • Infections
  • Wounds
  • Burns
  • Dermatitis
  • Rash

These formulations are either water or oil-based.

Parenteral medications

The name “parenteral” comes from para (next to) and enteral (GI system), meaning that drugs are given “outside” the GI-tract. It can be intraarterial, intracardiac, intradermal, intraarticular, intralesional, but most commonly intravenous, intramuscular or subcutaneous.

The advantages are that the onset of action can be nearly instant and that the absorption is 100% and avoids the first-pass effect.

Subcutaneous route

Drugs injected subcutaneously are absorbed slowly. It is used if we want the drug to have a sustained effect.

Irritating medications cannot be given subcutaneously as they would cause pain and necrosis. Drugs cannot be given in a high concentration either as it will cause abscess formation.

Intramuscular route

Drugs that are too concentrated or to irritant to be given subcutaneously can be given intramuscularly. The muscle tissue is more vascularized, so absorption is quicker than in a subcutaneous injection.

If we want to give a drug intramuscularly but also have a sustained effect we can prepare it as an aqueous suspension or oil, which causes the drug to be absorbed more slowly.

Intravenous route
Intravenous infusion set

Drugs are given intravenously when an immediate onset of action is desired. The resulting level of drug in the blood is very predictable and can be achieved very quickly.

We can give infusions intravenously as well. This is the best way to maintain a stable level of the drug in the blood.

Inhalational medication

Drugs that should be given directly into the airways are administered by an inhaler. This is used to treat asthma and chronic bronchitis. By injecting the drug directly into the airways, we can give a smaller dose of the drug and achieve fewer systemic side effects.

The drug is in liquid form in a specialized container and placed in a plastic thingy. When activated the drug is turned into an aerosol that should be breathed in by the patient.

Another type of inhaler is the dry powder inhaler, which is used when the drug is in a dry powder form.

Prescription writing

Prescription writing routines differ from country to country; however they should always contain at least these 8 points:

  1. Date
    • Prescriptions usually expire after some time
  2. Name, address, age of the patient
  3. R, Rp or Recipe written somewhere at the beginning
    • Recipe means “take” in Latin. As in, take this drug.
    • The right leg of the R is often crossed, so that it looks like Rx (see picture below)
  4. Name and strength of the drug
  5. Directions to the pharmacist
    • If the pharmacist should make “home-made” formulations
  6. Directions to the patient
    • Also called the label
    • Should be accurate and simple
    • “Take 10mg every fourth hour for five days”
  7. Refill information
    • Some countries allow physicians to prescribe prescriptions that can be re-used by the patients. In that case the physician can write so here
    • However, it often says “Do not refill” so that the patient can’t get more drugs than they need
  8. Signature, name, address of the doctor, stamp
Example of what a prescription should look like. Note the R

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