One of the most misunderstood concepts — even by professionals!
Let’s break it down in such a way that even your non-pharma friend will get it:
1. HALF-LIFE
This is about how long a drug remains active inside your body after you take it.
Think of it like this
If you take a tablet with a half-life of 6 hours,
It means every 6 hours, the amount of drug in your blood will become HALF.
So:
- 6 hr: 50%
- 12 hr: 25%
- 18 hr: 12.5%
- …and so on
Why it’s important?
- Tells you how often to take the medicine
- Short half-life → needs frequent dosing
- Long half-life → stays longer, fewer doses needed
- Also helps in designing sustained-release or controlled-release drugs
Fun fact:
Half-life is used to calculate when a drug gets fully out of your system
(usually takes 5–6 half-lives to be eliminated)
2. SHELF LIFE
Shelf life is NOT about what happens in your body.
It’s about how long a medicine remains stable and usable while it’s sitting on the shelf — even before opening it.
A tablet has a shelf life of 3 years →
It means: If stored properly (no humidity, no sunlight, proper temperature), it will remain chemically stable for 3 years from manufacturing.
Things that kill shelf life:
- Moisture
- High temperature
- UV light
- Air exposure
Why it’s important?
Medicines degrade slowly over time
They may look fine but might lose strength
If chemical bonds break — it can become toxic or useless
Fun fact:
Shelf life is determined by stability testing during drug development companies literally store it for months and years to test!
3. EXPIRY DATE
This is the last date you should consume a drug — period.
It’s printed based on the shelf life + safety margins.
So even if it looks good — after this date, you can’t trust its safety or strength.
Especially dangerous for:
- Antibiotics
- Injectables
- Liquid syrups
Why it matters?
After expiry, drug may become less effective or harmful
It’s a legal boundary — doctors, pharmacies, and manufacturers can get into trouble if they use or sell expired drugs
So, What’s the difference?
- Half-life: How long the drug stays active in your body. After taking the drug
- Shelf life: How long the drug stays stable on the shelf While storing the drug
- Expiry: Final date to use the drug safely & effectively. Legal limit for usage
The Bottom Line
- Half-life = In your body
- Shelf life = In the pack
- Expiry = Beyond this, it’s risk!
Read also: Same Drug With Different Dosage Form
Resource Person: Dev Soni