Generally, spice jars are not designed to be stored sideways and may leak, especially if the lids are not tightly sealed or if the spices are finely ground. It's best to store them upright.
Most spice jars are designed with a friction-fit or threaded lid intended for upright storage. When stored sideways, gravity can work against the seal, causing finely ground spices like paprika, turmeric, or chili powder to seep out. Even spices that appear coarser can gradually escape if the lid isn't perfectly airtight. The oil content in some spices can also contribute to leakage, as the oil can dissolve and seep through even small gaps. Furthermore, temperature fluctuations can cause the air inside the jar to expand and contract, potentially forcing spices out. To test your spice jars, fill them with a finely ground spice, seal them tightly, and lay them on their side for a few days to observe if any leakage occurs.
If you must store spice jars sideways (e.g., in a drawer), ensure the lids are tightly sealed and consider adding a layer of plastic wrap under the lid for an extra barrier. Regularly check for leaks to prevent a messy spice drawer.