Accidentally Left Doxycycline In A Hot Car

Answer

Thank you for reaching out, and great question!

Questions regarding the stability of prescription drugs exposed to extreme temperatures are ones we get fairly often.

While we can provide some guidance here, it's important to state upfront that for most drugs, it's challenging to determine if they have been compromised to the point of being ineffective after being exposed to extreme conditions for a short period. This difficulty arises due to the lack of methods to accurately assess the damage and, frankly, the limited data available for many drugs.

Having said that, for doxycycline, there have been thermostability studies published that give us a good indication that leaving a single dose in a hot car for a few hours likely will not greatly affect the drug.

What The Studies Say

One study I found (Thermostability testing and degradation profiles of doxycycline in bulk, tablets, and capsules by HPLC - PubMed (nih.gov)) tested the thermal degradation of doxycycline in several different dosage forms (tablets, capsules, and bulk powder) at various temperatures over 30 and 90 days.

The assessment was done by two different methods: HPLC (High-Performance Liquid Chromatography) and MEKC (Micellar Electrokinetic Chromatography). Understanding these methods is not really important for the purposes of this answer, but I wanted to define them so you can understand the data table below from the study:

You didn't mention the dosage form of doxycycline you have or how hot it was outside, but let's assume the most extreme conditions (capsules vs. tablets and the hottest temperature).

As you can see in the chart above, the study found that doxycycline capsules, when kept at 70 degrees Celsius (~156 degrees Fahrenheit) for 30 days, retained 87.32% of their labeled strength (based on the HPLC assay). So, for example, if the strength of your doxycycline was 100mg, after 30 days at 156 degrees Fahrenheit, 87.32mg would remain. Based on what I could find, it seems 160 degrees is around the maximum temperature the inside of a car could get to on a very hot, sunny day.

You stated that you left the drug in your car for a couple of hours. While I couldn't find stability data for this short period, it stands to reason that less time exposed to high temperatures would result in less degradation.

Thoughts

Overall, it's likely there was some degradation from your doxycycline being stored outside the recommended temperature range (which is 68-77 degrees Fahrenheit), but it likely isn't too significant since it was only for a few hours and the study referenced above showed about 13% degradation after 30 days at 156 degrees Fahrenheit.

Nevertheless, if you are concerned about the potency of your medication, I highly recommend reaching out to your doctor or pharmacy for specific advice regarding your medication situation.

Final Words

Thank you so much for reaching out, and please do so again anytime!