Stability, Storage Conditions and the Cold Chain
9 minute read
What you will learn
- Define stability and shelf life
- Describe chemical, physical and microbiological instability
- List the factors that affect stability and how to control them
- State the usual storage conditions
- Describe the cold chain and its equipment
- Calculate a simple shelf life
Key points
- Stability is the ability of a medicine to keep its quality, strength and safety until it is used.
- Instability can be chemical (breakdown), physical (change of appearance or form) or microbiological (germs).
- Temperature, moisture, light, oxygen, pH, germs, the container and time all affect stability.
- Shelf life is the time the product stays within its limits when stored correctly. It ends at the expiry date.
- Most products are kept cool and dry, away from light. Some need a refrigerator.
- The cold chain keeps vaccines and other products at 2 to 8 °C from the manufacturer to the patient.
- Cold-chain equipment includes refrigerators, cold boxes, vaccine carriers and thermometers.
Stability
Stability is the ability of a medicine to keep its quality (strength, appearance, safety and performance) from the time it is made until it is used. A medicine that is not stable may be weak, harmful or spoiled.
Types of instability
| Type | What happens | Examples |
|---|---|---|
| Chemical | The drug breaks down, so strength falls and harmful products may form | Hydrolysis (breakdown by water, as in many antibiotics) and oxidation (breakdown by oxygen, as in vitamin C) |
| Physical | The appearance or form changes | Tablets crumble or soften, creams separate, suspensions cake, emulsions crack, colour fades |
| Microbiological | Germs grow | Mould in syrups, creams and mixtures with weak preservatives |
Factors affecting stability
| Factor | Effect | Control |
|---|---|---|
| Temperature | Heat speeds up breakdown and may melt or soften products. Cold can freeze and break some | Store at the stated temperature |
| Moisture and humidity | Make powders cake, tablets soften and drugs break down | Keep dry, close tightly, use desiccant or foil |
| Light | Breaks down light-sensitive drugs and fades colour | Amber or opaque containers, dark storage |
| Oxygen (air) | Oxidises some drugs | Fill containers well, close tightly, use antioxidants in the formula |
| pH | Many drugs are stable only in a certain pH range | Use buffers, as the formula says |
| Germs | Spoil water-based products | Hygiene, preservatives, sterile products |
| Container | The drug may react with it, stick to it, or be absorbed | Use the container recommended for the product |
| Time | The longer the storage, the more breakdown | Use FEFO, and respect expiry dates |
| Handling | Rough handling breaks packs | Careful transport |
Shelf life and expiry
The shelf life is the time during which a product stays within its quality limits when stored correctly. It ends on the expiry date printed on the pack. A common standard is that a medicine should keep at least 90% of its labelled strength until expiry.
Manufacturers set shelf life through stability studies. In accelerated testing, products are stored at higher temperatures and humidity to see how quickly they break down, and the result is used to predict shelf life, which is then confirmed in real-time studies.
Worked example
A product loses 2% of its labelled strength each month, steadily. It must keep at least 90%.
- Allowed loss = 100 − 90 = 10%
- Shelf life = 10 ÷ 2 = 5 months
Real breakdown is not always steady, so this is a simple example only.
Storage conditions
Typical conditions on labels:
| Description | Meaning (typical) |
|---|---|
| Store in a cool place | A cool place without direct heat |
| Store at room temperature | Normal controlled room temperature, not in direct heat |
| Store in a refrigerator | 2 to 8 °C |
| Do not freeze | Freezing damages the product |
| Protect from light | Keep in the original, light-proof pack |
| Keep dry | Keep in a tight container, away from damp |
The exact temperature limits come from the pharmacopoeia and the product label. In hot climates such as Nigeria, a cool, shaded, ventilated store, and a refrigerator for the products that need it, are important. Check the temperature daily with a thermometer.
The cold chain
The cold chain is the system of keeping products at the right low temperature at every stage, from the manufacturer to the patient. It is essential for vaccines, insulin, some antibiotics, biologicals and some other products.
Manufacturer → Airport or port → Central cold store → State store → Local store → Health facility → Patient
A single break in the chain can ruin the product, and the damage is often invisible.
Equipment
| Equipment | Purpose |
|---|---|
| Cold room | Large-scale storage at central and state level |
| Refrigerator (including ice-lined and solar types) | Storage at health facilities, at 2 to 8 °C |
| Freezer | For freezing ice packs, and for the few products stored frozen |
| Cold box | Transport of larger amounts, with ice packs |
| Vaccine carrier | Small insulated box for outreach, with ice packs |
| Ice packs | Keep the carrier cold. They must be properly conditioned before use |
| Thermometers and temperature records | Show that the temperature stayed in range |
| Vaccine vial monitor (VVM) | A label on the vial that changes colour as heat exposure builds up |
Good practice
- Check and record temperatures at least twice a day.
- Keep vaccines in the middle of the refrigerator, not in the door or against the walls.
- Do not store food or drinks in the vaccine refrigerator.
- Do not freeze vaccines that must not be frozen.
- Keep a plan for power cuts, with spare ice packs, a generator or solar power.
- Use FEFO, and check VVMs.
- Never use a product that is suspected of having been out of range. Report it, and follow the guidance.
Exam focus
- List the factors that affect the stability of medicines.
- Define shelf life and expiry date.
- Describe the cold chain and name its equipment.
- Common mistake: freezing vaccines, which damages many of them.
Quick check
Test yourself before you move on. Pick an answer to see the explanation.
1.Shelf life is:
2.Which of these affects the stability of a medicine?
3.What temperature range is usually used in the vaccine cold chain?
4.A product loses 2% of its labelled strength each month. If it must keep at least 90%, how long is its shelf life, assuming a steady loss?
5.Which device is used to carry vaccines for outreach with ice packs?