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Exposing medication to extreme heat

And its chemical effects


The majority of us look forward to when summer is just around the corner. It is a time for parents to start planning days off to be able to go on holiday with their kids to relax from their studies and enjoy sunsets at the beach. But for people who take medication, whether this just be a week-long course of antibiotics or for long-term conditions, summer may also be a chance for some negligence to occur. Specifically, alongside making sure you have applied SPF to protect your skin from the sun’s rays, you should also protect your medicine as well. This applies to both oral and non-oral drugs. Experts at The Montreal Children’s Hospital say that “many prescription drugs are very sensitive to changes in temperature and humidity”; in this article, we will therefore discuss the effect of extreme heat on drugs from a medicinal chemistry perspective.

Factors affecting drug activity due to heat

Certain drugs may begin to degrade before their expiry date if not stored appropriately. This affects the efficacy, which is the maximum biological response that is achievable with a certain drug. A dose-response curve can be plotted (see Figure 1) to show the relationship between the two variables; the label Emax refers to the efficacy. During hot weather, the structure of the drug can change and therefore unable to bind to its target, causing a lowered and shifted Emax to be seen. Simply put, the medication will not relieve your symptoms as effectively.

Another physiochemical property of a drug that can be altered in the heat is the potency. Many people confuse this term with efficacy, but potency refers to the concentration of a drug required to achieve 50% of its maximum therapeutic effect i.e., half the Emax. Potency is therefore also known as EC50, which abbreviates for ‘half maximal effective concentration’. The lower the concentration needed, the more potent your drug is. Like reduced efficacy, the drug’s potency will also decrease in the heat due to altered chemical structure. For drugs like antibiotics, it is crucial to note that if potency is reduced significantly, it could risk infection spreading to other parts of the body as the medication will not fight off bacteria as well as it should. Potentially dangerous!

Finally, drug absorption is when a drug moves into the bloodstream after being administered. The chemical structure of the drug and the environment in which it is present hugely affects this; for example, if a lipophilic (‘fat loving’) drug is also present in a lipophilic surrounding, fast absorption is seen as they work well with each other. As you have probably guessed, high temperatures outside of the body can reduce drug absorption due to the above factors mentioned, as the drug is not in its optimal structure to be absorbed effectively.

Examples of medicine that are heat sensitive

Here is a list of some medicines that require extra care to prevent the above:

1)     Nitroglycerin – used to treat chest pains for those with cardiovascular disease. It is especially sensitive to heat or light as it degrades very fast. Dr. Sarah Westberg, a professor at The University of Minnesota College of Pharmacy, says you should follow the storage instructions and replace them regularly.


2)     Some antibiotics – research has shown that ampicillin, erythromycin, and furosemide show a reduction in activity in the heat, although this was found after storing them for a year in a car with a temperature exceeding 25 degrees Celsius. Other antibiotics such as cefoxitin are shown to have some “stability in warmer climates”.


3)     Levothyroxine – used to treat an underactive thyroid, also known as hypothyroidism. This drug should be stored between 15 to 30 degrees Celsius, although even 30 is quite high so the lower the temperature the better. Interestingly, levothyroxine isn’t heat sensitive itself, it is the fact that the body becomes sensitive to the drug and may make a person feel strange in the heat.


4)     Metoprolol succinate – used to treat high blood pressure, also known as hypertension, and heart failure in emergencies. The ideal storage conditions for this drug are 15 to 30 degrees Celsius, like Levothyroxine.

Key things to look out for with your medicine in the heat

Below are the 2 main things you should be checking for before taking your medicine in the summer:

1)     Change in colour – Light can initiate all sorts of reactions, such as oxidation. If, for example, your medicine that is normally white has now changed into a different colour, this suggests that a reaction has taken place within your drug and will not be effective when administered.


2)     Change in texture – Similar to change in colour, if a normally solid, oral tablet has become soft then this also suggests that the medication will not be as effective when consumed.

How you can prevent your medicine from degrading

To make sure you do not contribute to wasting medicine, you should do the following:

1)     Check storage information – for any medication that you take, this will let you know how to store them correctly.


2)     Travel with care – do not pack prescription drugs into your luggage, as it will almost always become very warm due to the surrounding environment. Instead, carry your medicine with you with the labels still on.


3)     Do not leave medicine in any vehicle – everyday vehicles such as cars tend to get warm after a period, which can affect the colour and texture of your medicine.


4)     Careful deliveries – for those who have their medicine delivered to them, you can request for your local pharmacy to deliver your medicine in temperature-controlled packages.


As discussed, chemicals in the majority of over-the-counter prescription drugs are heat sensitive and should therefore be handled with care, to prevent degradation of the drug. Changes in colour and texture are signs of degradation, which result in loss of efficacy, absorption, and potency. However, many other pharmacological factors interfere, so scientists especially involved in drug synthesis should (or continue to) take great precautions with the manufacturing process. Drugs are costly to make and require a lot of time, so the takeaway is to store them correctly! You should contact your pharmacist if you are still unsure about your prescription(s).

Written by Harsimran Kaur Sarai

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