A in his email specifies to store it in a drawer, but I found this:
Nembutal package insert / prescribing information for healthcare professionals. Includes: indications, dosage, adverse reactions and pharmacology.
www.drugs.com
"Exposure of pharmaceutical products to heat should be minimized. Avoid excessive heat. Protect from freezing. It is recommended that the product be stored at 20° - 25°C (68° - 77°F), however, brief excursions are permitted between 15° - 30°C (59° - 86°F). See USP controlled room temperature."