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You have a few options when it comes to spicy ketchup. You can easily make it yourself (see our recipe for smoky chipotle ketchup) or you can buy one of the many options available at most grocery stores. Heinz also carries a chili sauce in its line (affiliate link) that’s really much more like spicy ketchup than a Sriracha.

When substituting, use common sense. Cayenne pepper is as much as 50 times hotter than sweet paprika, so a 1:1 substitution wouldn't work unless you want some heat—the right substitution, in this case, would be 1/3 of a teaspoon of cayenne per 1 teaspoon of paprika. Make an informed decision, but also use thorough recipes that might suggest reasonable substitutions.