Leaving medicine in your car is generally a bad idea, especially in warm weather. A parked car can reach 100°F inside within 30 minutes on a mild summer day, and temperatures keep climbing from there. Most medications are designed to be stored between 59°F and 86°F, which means a parked car in direct sunlight can exceed safe storage limits surprisingly fast.
How Hot Cars Actually Get
The National Weather Service tracked interior car temperatures on a day when the outside air was only in the low 90s. The car’s interior hit 100°F in just 30 minutes, 110°F within an hour, and peaked above 120°F after about two hours. Opening the windows afterward only dropped it to 112°F. On hotter days, or in direct sun in places like Arizona or Texas, interior temperatures can climb even higher. Shade helps somewhat, but a shaded car still traps heat.
This matters because the two main chemical reactions that break down medications, oxidation and hydrolysis, both accelerate with heat. Higher temperatures speed up these reactions, meaning a drug that would stay potent for months on your bathroom shelf can degrade meaningfully after hours in a hot car.
Which Medications Are Most Vulnerable
Insulin
Insulin is one of the most heat-sensitive medications people commonly carry. It should be refrigerated at home (36°F to 46°F) and can be kept at room temperature for up to 28 days. But insulin used in a pump should be discarded if exposed to temperatures above 98.6°F, a threshold a parked car blows past in minutes on a warm day. Cornell University’s guidance is blunt: do not store insulin in a car. Heat-damaged insulin doesn’t look obviously different, but it loses its ability to lower blood sugar effectively. Freezing is equally destructive, potentially cracking vials and allowing bacteria in.
Emergency Contraception
Plan B and similar emergency contraception pills need to stay below about 77°F to 80°F to maintain effectiveness. They can tolerate brief transport between 59°F and 86°F, but a car interior that hits 120°F is well beyond those limits. This is a medication where reduced potency has serious real-world consequences.
Liquid Antibiotics
Reconstituted liquid antibiotics, the kind commonly prescribed for children’s ear infections and respiratory infections, are particularly fragile. One key ingredient in a widely used combination antibiotic lost 72% of its potency after seven days at just 82°F (28°C), compared to only about 13% loss when properly refrigerated. At 95°F, roughly half of that ingredient degraded in just seven hours. If you’re picking up a liquid antibiotic from the pharmacy and running other errands, even a short stop with the medication sitting in a hot car could start undermining the treatment.
Inhalers
Pressurized metered-dose inhalers like albuterol carry a physical risk beyond just drug degradation. The pressurized canister can leak or even burst at very high temperatures. The NIH warns against exposing inhalers to high heat or flames. A car dashboard in July is exactly the kind of environment that creates problems.
Epinephrine Auto-Injectors
EpiPens and similar devices are a slightly more nuanced case. Research shows that brief heat exposure doesn’t significantly degrade epinephrine, and the injection devices still fire correctly after temperature swings. However, prolonged or constant heat does cause the drug to break down. If you carry an auto-injector for a severe allergy, occasional short trips through heat are less concerning than leaving it in your glove compartment all summer.
What About Short Errands?
A quick 10-minute stop at the grocery store on a 75°F day is unlikely to destroy most solid medications like tablets and capsules. The concern grows with higher outside temperatures, longer durations, and more sensitive formulations. Solid pills are generally more heat-stable than liquids, creams, suppositories, and biologics. But “short errand” thinking adds up: repeated cycles of heating and cooling stress medications even if no single episode seems extreme.
If you must leave medication briefly, put it in an insulated bag or cooler (without direct ice contact for things like insulin). The trunk stays slightly cooler than the passenger cabin since it gets less direct sunlight through glass, though it still gets hot. The glove compartment and dashboard are the worst spots.
Cold Weather Concerns
Winter poses its own risks. Insulin can freeze and lose potency. Liquid medications can crystallize or separate in ways that alter dosing. Pressurized inhalers may not spray properly when very cold. If overnight temperatures drop below freezing where you park, your car is essentially a refrigerator or freezer, and not the carefully controlled kind your medications need.
How to Tell If Medication Has Been Damaged
Sometimes you can spot heat damage: tablets that have changed color, capsules that are stuck together, creams that have separated, suppositories that melted and resolidified into odd shapes, or liquids that look cloudy when they shouldn’t. But many degraded medications look completely normal. Insulin, epinephrine, and birth control pills can lose effectiveness without any visible change. If you realize a medication spent hours in a hot or freezing car, the safest approach is to replace it, particularly for anything where reduced potency carries real health risks.
The FDA’s straightforward advice: don’t leave medications in the car in summer or winter. For everyday carry, bring your medications inside with you. For road trips, pack them in an insulated bag in the passenger cabin rather than the trunk, and bring them into your hotel room at every stop.