Four things degrade medicines: heat, which is the biggest factor, then moisture, light and oxygen. A common rule of thumb has each 10 degrees C rise roughly doubling the rate at which many drugs break down, so a medicine kept at 40 degrees C ages about four times as fast as one kept at 20 degrees C. Because lost strength cannot be seen, a normal-looking tablet may still be weak, while a discoloured, crumbling, sticky or foul-smelling one is suspect.
Heat, moisture, light and oxygen decide how fast a medicine breaks down, so its history matters as much as its date.
Heat Is The Largest Single Factor
Chemical breakdown speeds up as temperature rises. A widely used rule of thumb says that for many chemical reactions the rate roughly doubles for every 10 degrees C. It is an approximation for many reactions, not a measured property of every drug, but it explains why a hot car, an attic or a sunlit window destroys medicine far faster than a cellar.
Worked Example
Compare storage at 20 degrees C and at 40 degrees C.
The difference is 40 - 20 = 20 degrees C, which is two steps of 10 degrees C. Each step doubles the rate, so 2 x 2 = 4. A bottle held at 40 degrees C would be ageing about four times as fast as one held at 20 degrees C, by this rule of thumb. The result is an estimate, not a measurement of any particular drug.
The Other Three
- Moisture. Water activates reactions that break down active compounds, and humidity speeds them. That is why bottles contain silica gel packets, which should stay in the bottle and never be eaten. Aspirin that smells of vinegar has been breaking down in moisture.
- Light. Ultraviolet light breaks molecular bonds, which is why many bottles are amber or brown. Keep medicine in opaque containers away from windows.
- Oxygen. Oxidation degrades many compounds. Keep containers sealed, and open them as little as possible. Once a bottle is opened its contents are exposed to air and moisture each time.
Method: Judging A Bottle
Step 1: Ask where it has been. A bottle that has spent months in a car, an attic, a bathroom or on a sunny sill has aged faster than the date suggests. Treat it as older than printed.
Step 2: Look at the contents. Tablets that are crumbling, cracked, discoloured, spotted, sticky or fused together are suspect. Capsules that are soft, stuck together or deformed are suspect.
Step 3: Smell it. A strong vinegar smell from aspirin, or any strong odour that is not normal for the product, is a warning.
Step 4: Check liquids and injectables. Cloudiness, colour change or particles in a liquid that should be clear mean it should not be used.
Step 5: Accept the limits. None of these checks measures potency. A normal-looking medicine can still be weak, so rely on storage history first.
Common Mistakes
- Judging by looks alone. Potency loss is invisible.
- Throwing out the silica gel. It protects the contents.
- Keeping medicine in the bathroom. Heat and steam combine to age it faster.
Safety
Broken-down medicine can be weak or harmful, and you cannot tell by looking
Most degraded solid tablets are simply weaker, but some medicines produce harmful breakdown products, and a weaker antibiotic or emergency drug can fail when it matters. If a medicine looks wrong or smells wrong, do not use it. Amounts to take come from a reference, not from this article.
What’s Next
- Learn next: Medications That Genuinely Expire — the cases where decay is harmful
- Combine with: Optimal Storage Conditions — controlling the four factors