diff --git a/src/p_effect.h b/src/p_effect.h
index 614702a49..eb417d63c 100644
--- a/src/p_effect.h
+++ b/src/p_effect.h
@@ -31,6 +31,8 @@
 **
 */
 
+#pragma once
+
 #include "vectors.h"
 
 #define FX_ROCKET			0x00000001