Why medicine tablets are sealed behind aluminum foil: The metal blocks moisture, light, and oxygen that can slowly damage drugs

Ever popped a pill out of its foil-and-plastic wrap and wondered why it’s wrapped so tight? It's not just for show. A 2025 study titled ‘Predictive Modeling of Drug Product Stability in Pharmaceutical Blister Packs,’ in the journal Pharmaceutics, found that tablets stored in packs that possess better moisture barrier properties experience lower degradation rates compared to those stored in less efficient packages.

The tablet in your hand is more fragile than it looks

While a tablet might seem solid and stable, it actually isn't quite that simple. Almost all pills are, in fact, highly sensitive chemical compounds, which might easily degrade through exposure to such factors as moisture, oxygen, and even light, which can degrade the drug's effectiveness in several ways. This is exactly why manufacturers cannot just fill bottles with their products and call it a day; they need to test how the particular drug reacts to its environment and find packaging that protects against it.


Moisture is enemy number one for most drugs

Water vapor is one of the biggest reasons medicines lose potency. Even the tiny amount of moisture in normal air can slowly seep into a packet and interact with the active ingredient.

Researchers testing this directly in a study titled ‘The effects of packaging on the stability of a moisture-sensitive compound,’ published in the International Journal of Pharmaceutics, measured the amount of water passing through the different blister materials each day. Plain PVC plastic blisters let through about 0.259 milligrams of moisture per blister per day, while cold-formed aluminum blisters let through only about 0.001 milligrams; aluminum let through roughly 259 times less moisture. In theory, that difference seems tiny, but over months of storage it can decide whether a tablet stays active until the printed expiry date or starts to break down early. That is why some medicines, such as effervescent tablets or some antibiotics, are packed in all-aluminum 'alu-alu' packs rather than clear plastic blister packs like those used for everyday painkillers. The drugs inside are more sensitive to humidity, so they need a tougher shield.

Light can quietly alter a drug's structure

Some medicines may also be photosensitive, meaning exposure to light can cause chemical reactions within the substance. It’s not just direct sunlight, either; the fluorescent light in a pharmacy or your bathroom can contribute over time. The review, ‘Photostability and Photostabilization of Drugs and Drug Products,’ published in the International Journal of Photoenergy in 2016, shows that light can alter drug molecules, reduce efficacy, and sometimes create degradation products with different biological properties. The review also notes that global pharmacopoeias recommend specifically light-resistant containers and protected storage for drugs known to be light-sensitive. One of the best materials for blocking light out completely is aluminum foil since it is essentially opaque.

Oxygen plays its part too

While oxygen is discussed less often than moisture and light, it can also cause problems. This happens due to the fact that some molecules of drugs are oxidized by oxygen and lose their effectiveness because of the changes in their structure. Vitamins, some hormones, and antibiotics are especially vulnerable to these changes. Aluminum foil is also an excellent oxygen barrier, blocking gas exchange far more effectively than plastic. That is why this material is used in the packaging of many kinds of drugs, not just a few.

Why aluminum beats plain plastic

Plastic blister packs are cheaper and easier to make, but by themselves they offer nowhere near the same level of protection. The study above shows that regular PVC plastic slowly allows moisture, gases, and some light to pass through. That is fine for chemically stable drugs, regardless of whether they're prescription or over-the-counter; the packaging choice comes down to how sensitive the specific drug is, not its OTC status. For anything more sensitive, manufacturers add a layer of aluminum foil, either as the lid of the blister or as its entire shell. The metal is thin and light, doesn’t interact with the drug, and shuts out moisture, light, and oxygen. That combination cannot be obtained from plastic alone.

The takeaway for the next time you pop a pill

Your medication’s foil packaging is more than a design choice; it is selected based on tests of how the drug reacts to its surroundings. So, the next time you think about removing a week’s supply of tablets from their blister packs and storing them without protective packaging, remember that you may be reducing their protection. If you really need to carry loose tablets, try to use them quickly and keep them away from heat, humidity, and direct light.