We get this question every summer. The less heat the better is the general rule. A few days in hot weather while in transit will not impact the products potency. Long distance deliveries are by air which is no problem. Only the "last mile" is vulnerable to higher temps. However, leaving the package on your doorstep in 90+ degree weather, especially in direct sunlight can affect the product potency and even the safety seal. Make sure to collect it as soon as possible if the delivery man leaves it outside, or use an address where you know someone will be available to receive it in person.