Dosage Forms and Their Classification
9 minute read
What you will learn
- Define a dosage form
- State the components of an ideal dosage form
- Classify dosage forms as solid, liquid (including parenteral), semi-solid, and gases and aerosols
- Define and give examples of the common dosage forms
- Name the factors that influence the choice of a dosage form
Key points
- A dosage form is the physical form in which a medicine is made and given to the patient, such as a tablet, syrup or cream.
- Every dosage form has two kinds of ingredient: the active ingredient (the drug) and the excipients (everything else).
- Dosage forms are classified by physical state: solid, liquid, semi-solid, and gases and aerosols.
- Parenterals are dosage forms given by injection. They must be sterile.
- Ointments, creams, pastes and gels are semi-solid dosage forms for the skin and other surfaces.
- The best dosage form depends on the patient, the drug and the condition being treated.
What is a dosage form?
A dosage form is the physical form in which a medicine is made, supplied and given to the patient. A tablet, a syrup, a cream and an injection are all dosage forms.
The same drug is often available in more than one dosage form. Paracetamol, for example, comes as tablets, syrup, suspension and suppositories. The right one depends on who the patient is and how they need to take it.
Why not give the drug on its own?
A pure drug is rarely given directly, because:
- the dose is often tiny (a few milligrams) and impossible to measure accurately by hand;
- it may taste bad, irritate the tissues or break down quickly;
- it needs to be in a form that suits the route, such as swallowing, injecting or applying to the skin.
The dosage form solves these problems.
Components of an ideal dosage form
Every dosage form has two parts:
- The active ingredient, the drug that treats the condition.
- The excipients, all the other ingredients that make the product usable.
| Excipient | Job | Example |
|---|---|---|
| Diluent or filler | Adds bulk | Lactose |
| Binder | Holds a tablet together | Starch paste |
| Vehicle or solvent | Carries the drug in a liquid | Purified water |
| Preservative | Stops microbes growing | Methyl hydroxybenzoate |
| Sweetener and flavour | Improve taste | Sucrose |
| Colour | Appearance and identification | Approved dyes |
| Stabiliser | Keeps the product from breaking down | Antioxidants |
An ideal dosage form delivers the correct dose accurately, is stable, safe, easy to use and acceptable to the patient.
Classification by physical state
| Class | Examples |
|---|---|
| Solid | Powders, granules, tablets, capsules, suppositories, pessaries, lozenges |
| Liquid | Solutions, syrups, elixirs, linctuses, mixtures, suspensions, emulsions, drops, lotions, mouthwashes, injections |
| Semi-solid | Ointments, creams, pastes, gels |
| Gases and aerosols | Inhalers, sprays, medical gases |
Solid dosage forms
These are shaped solids. Powders, granules, tablets and capsules are swallowed. Suppositories and pessaries are inserted into the rectum or vagina. Lozenges are dissolved slowly in the mouth. The earlier lessons in BDT 152 describe each in detail.
Liquid dosage forms
| Form | Definition |
|---|---|
| Solution | A clear liquid in which the drug is dissolved |
| Syrup | A concentrated solution of sugar in water, usually with a drug and flavour |
| Elixir | A clear, sweetened solution containing water and alcohol |
| Linctus | A thick, sweet liquid for cough and sore throat, sipped slowly and not washed down with water |
| Mixture | An oral liquid in a bottle with one or more drugs dissolved or suspended in a vehicle |
| Suspension | Fine solid particles spread through a liquid. Shake before use |
| Emulsion | Two liquids that do not mix (such as oil and water), with one spread through the other as tiny droplets |
| Drops | Liquid given by the drop into the eye, ear or nose, or by mouth |
| Lotion | A liquid for application to the skin without rubbing |
| Mouthwash and gargle | Liquids used to rinse the mouth or throat and then spat out |
Parenteral dosage forms are liquids (or powders made into liquids) given by injection into the body, bypassing the gut. They include injections given into a vein (intravenous), a muscle (intramuscular) and under the skin (subcutaneous). Parenterals must be sterile and free from particles.
Semi-solid dosage forms
| Form | Definition |
|---|---|
| Ointment | A greasy semi-solid for the skin. It stays on the skin and forms a protective layer |
| Cream | A semi-solid emulsion. Less greasy than an ointment, spreads easily and washes off with water |
| Paste | A stiff ointment that contains a large amount of powder, such as zinc oxide paste. It stays where it is put and protects the skin |
| Gel | A clear or translucent semi-solid made with a gelling agent, often water-based |
Gases and aerosols
An aerosol is a product in a pressurised container that releases the drug as a fine mist or spray. Examples are asthma inhalers and throat sprays. Medical gases, such as oxygen, are also in this group.
Classification by route
Dosage forms can also be grouped by how they are given:
| Route | Examples |
|---|---|
| Oral (by mouth) | Tablets, capsules, syrups, mixtures |
| Topical (on the skin) | Creams, ointments, lotions |
| Parenteral (by injection) | Injections |
| Rectal | Suppositories |
| Vaginal | Pessaries |
| Inhalation | Inhalers |
| Ocular and otic | Eye drops, ear drops |
Choosing a dosage form
The choice depends on:
- the age of the patient: syrups for small children, for example;
- whether the patient can swallow;
- how fast the effect is needed: an injection works faster than a tablet;
- where the drug must act: a cream for a skin infection;
- whether the drug is stable in that form;
- what the patient prefers, and what is available and affordable.
Exam focus
- Questions often ask you to place a named dosage form into its correct class. Learn the table below.
- Know the difference between a solution (dissolved), a suspension (particles spread through the liquid) and an emulsion (two liquids).
- Know the difference between an ointment (greasy), a cream (emulsion), a paste (stiff, high in powder) and a gel.
- Common mistake: forgetting that excipients are part of every dosage form, not just the drug.
Quick check
Test yourself before you move on. Pick an answer to see the explanation.
1.Which of these is a semi-solid dosage form?
2.The non-drug ingredients in a dosage form are called:
3.Which dosage form must be sterile?
4.A suspension is a liquid in which:
5.An inhaler is classified as: