Cayenne pepper is a well-known way to add moderate heat to dishes and is widely available in powdered form. If your dish calls for cayenne powder and you are out, you have many easy-to-find options that can bring the heat to your dish, but note – many do so with extra flavor notes that can change the profile of your meal.
Your best bet: Crushed red pepper or gochugaru
Crushed red pepper often uses cayenne pepper as a base (along with a few other chilies), so you can ground the red pepper flakes down into a powder for a decent cayenne pepper substitute – though it will never be as hot. Gochugaru is ground Korean red pepper consisting of red chilies that have been dried in the sun. While not as hot as cayenne peppers (10,000 Scoville heat units compared to 30,000 – 50,000 for cayenne powder), these chilies still pack plenty of heat. They also have a delightful red color that should stand out in your dish.
A decent second choice: Hot Spanish paprika
Hot Spanish paprika is not quite as hot as cayenne powder, but you can compensate for that by adding more of it. Its appearance is very similar to that of cayenne powder, allowing it to imbue dishes with a similar red hue. If the reason that you need a substitute is that cayenne pepper is too hot, pimenton is ideal since you have options that range from mild to hot. Note that pimenton is smoked, so you will have to account for that when deciding whether to use it in a dish. On the bright side, the smokiness should complement the other flavors in a recipe that calls for cayenne.
In a pinch: Jalapeño powder or chipotle powder
Both of these chilies work, but they have distinct flavor profiles compared to the more neutral flavor of cayenne pepper powder. Consider their use accordingly.
The jalapeño is arguably the best-known chili pepper in the United States due to its relatively low heat level and high level of flavor. Jalapeño powder make an excellent low-heat substitute for fresh cayenne peppers and should work well in most of the dishes that call for fresh cayennes, though there is a brightness to the flavor which is quite different than cayenne.
Chipotle peppers are jalapeños that have been dried and smoked. These peppers are often sold in powder form where they provide a gentler heat along with an earthy, smoky flavor. This flavor is richer than that of cayenne powder but can add increased depth to recipes that call for cayenne. Add in increments and increase to taste.
Other alternatives
Tabasco sauce can be an effective alternative to cayenne pepper and is easy to add to your dish in a hurry. It offers a similar red color, but is not quite as hot. Note that Tabasco sauce contains vinegar, which means that it can make your dish more acidic.