diff --git a/cmake/ci.cmake b/cmake/ci.cmake
index 4d89d901b..22cf09093 100644
--- a/cmake/ci.cmake
+++ b/cmake/ci.cmake
@@ -525,6 +525,7 @@ add_custom_target(ci_test_noglobaludls
     COMMAND CXX=${CLANG_TOOL} ${CMAKE_COMMAND}
     -DCMAKE_BUILD_TYPE=Debug -GNinja
     -DJSON_BuildTests=ON -DJSON_FastTests=ON -DJSON_GlobalUDLs=OFF
+    -DCMAKE_CXX_FLAGS=-DJSON_TEST_NO_GLOBAL_UDLS
     -S${PROJECT_SOURCE_DIR} -B${PROJECT_BINARY_DIR}/build_noglobaludls
     COMMAND ${CMAKE_COMMAND} --build ${PROJECT_BINARY_DIR}/build_noglobaludls
     COMMAND cd ${PROJECT_BINARY_DIR}/build_noglobaludls && ${CMAKE_CTEST_COMMAND} --parallel ${N} --output-on-failure
diff --git a/tests/src/unit-class_parser.cpp b/tests/src/unit-class_parser.cpp
index df821beb0..e5b7f5db8 100644
--- a/tests/src/unit-class_parser.cpp
+++ b/tests/src/unit-class_parser.cpp
@@ -11,7 +11,9 @@
 #define JSON_TESTS_PRIVATE
 #include <nlohmann/json.hpp>
 using nlohmann::json;
-using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#ifdef JSON_TEST_NO_GLOBAL_UDLS
+    using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#endif
 
 #include <valarray>
 
diff --git a/tests/src/unit-deserialization.cpp b/tests/src/unit-deserialization.cpp
index f4a76b303..d42a6997a 100644
--- a/tests/src/unit-deserialization.cpp
+++ b/tests/src/unit-deserialization.cpp
@@ -11,7 +11,9 @@
 
 #include <nlohmann/json.hpp>
 using nlohmann::json;
-using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#ifdef JSON_TEST_NO_GLOBAL_UDLS
+    using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#endif
 
 #include <iostream>
 #include <iterator>
diff --git a/tests/src/unit-element_access2.cpp b/tests/src/unit-element_access2.cpp
index ab59461eb..918c263d2 100644
--- a/tests/src/unit-element_access2.cpp
+++ b/tests/src/unit-element_access2.cpp
@@ -10,7 +10,9 @@
 #include "doctest_compatibility.h"
 
 #include <nlohmann/json.hpp>
-using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#ifdef JSON_TEST_NO_GLOBAL_UDLS
+    using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#endif
 
 TEST_CASE_TEMPLATE("element access 2", Json, nlohmann::json, nlohmann::ordered_json)
 {
diff --git a/tests/src/unit-json_patch.cpp b/tests/src/unit-json_patch.cpp
index 0b39d00ae..1f6716bde 100644
--- a/tests/src/unit-json_patch.cpp
+++ b/tests/src/unit-json_patch.cpp
@@ -10,7 +10,9 @@
 
 #include <nlohmann/json.hpp>
 using nlohmann::json;
-using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#ifdef JSON_TEST_NO_GLOBAL_UDLS
+    using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#endif
 
 #include <fstream>
 #include "make_test_data_available.hpp"
diff --git a/tests/src/unit-json_pointer.cpp b/tests/src/unit-json_pointer.cpp
index 445b8f026..4f457d0ee 100644
--- a/tests/src/unit-json_pointer.cpp
+++ b/tests/src/unit-json_pointer.cpp
@@ -11,7 +11,9 @@
 #define JSON_TESTS_PRIVATE
 #include <nlohmann/json.hpp>
 using nlohmann::json;
-using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#ifdef JSON_TEST_NO_GLOBAL_UDLS
+    using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#endif
 
 #include <sstream>
 
diff --git a/tests/src/unit-merge_patch.cpp b/tests/src/unit-merge_patch.cpp
index 47bab58f7..af56b0dec 100644
--- a/tests/src/unit-merge_patch.cpp
+++ b/tests/src/unit-merge_patch.cpp
@@ -10,7 +10,9 @@
 
 #include <nlohmann/json.hpp>
 using nlohmann::json;
-using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#ifdef JSON_TEST_NO_GLOBAL_UDLS
+    using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#endif
 
 TEST_CASE("JSON Merge Patch")
 {
diff --git a/tests/src/unit-msgpack.cpp b/tests/src/unit-msgpack.cpp
index 5c720f6ac..f90190315 100644
--- a/tests/src/unit-msgpack.cpp
+++ b/tests/src/unit-msgpack.cpp
@@ -10,7 +10,9 @@
 
 #include <nlohmann/json.hpp>
 using nlohmann::json;
-using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#ifdef JSON_TEST_NO_GLOBAL_UDLS
+    using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#endif
 
 #include <fstream>
 #include <sstream>
diff --git a/tests/src/unit-readme.cpp b/tests/src/unit-readme.cpp
index 71ffffff6..108cdd039 100644
--- a/tests/src/unit-readme.cpp
+++ b/tests/src/unit-readme.cpp
@@ -10,7 +10,9 @@
 
 #include <nlohmann/json.hpp>
 using nlohmann::json;
-using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#ifdef JSON_TEST_NO_GLOBAL_UDLS
+    using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#endif
 
 #include <deque>
 #include <forward_list>
diff --git a/tests/src/unit-regression1.cpp b/tests/src/unit-regression1.cpp
index 77a194db6..c7720b4f1 100644
--- a/tests/src/unit-regression1.cpp
+++ b/tests/src/unit-regression1.cpp
@@ -14,7 +14,9 @@
 #define JSON_TESTS_PRIVATE
 #include <nlohmann/json.hpp>
 using nlohmann::json;
-using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#ifdef JSON_TEST_NO_GLOBAL_UDLS
+    using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#endif
 
 #include <fstream>
 #include <sstream>
diff --git a/tests/src/unit-regression2.cpp b/tests/src/unit-regression2.cpp
index 5ba3d310f..3f82301cc 100644
--- a/tests/src/unit-regression2.cpp
+++ b/tests/src/unit-regression2.cpp
@@ -22,7 +22,9 @@
 #include <nlohmann/json.hpp>
 using json = nlohmann::json;
 using ordered_json = nlohmann::ordered_json;
-using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#ifdef JSON_TEST_NO_GLOBAL_UDLS
+    using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#endif
 
 #include <cstdio>
 #include <list>
diff --git a/tests/src/unit-udl.cpp b/tests/src/unit-udl.cpp
index 19d901f2b..526e05b27 100644
--- a/tests/src/unit-udl.cpp
+++ b/tests/src/unit-udl.cpp
@@ -8,8 +8,6 @@
 
 #include "doctest_compatibility.h"
 
-#undef JSON_USE_GLOBAL_UDLS
-#define JSON_USE_GLOBAL_UDLS 0
 #include <nlohmann/json.hpp>
 
 TEST_CASE("user-defined string literals")
@@ -48,4 +46,12 @@ TEST_CASE("user-defined string literals")
         CHECK(R"({"foo": "bar", "baz": 42})"_json == j_expected);
         CHECK("/foo/bar"_json_pointer == ptr_expected);
     }
+
+#ifndef JSON_TEST_NO_GLOBAL_UDLS
+    SECTION("global namespace")
+    {
+        CHECK(R"({"foo": "bar", "baz": 42})"_json == j_expected);
+        CHECK("/foo/bar"_json_pointer == ptr_expected);
+    }
+#endif
 }
diff --git a/tests/src/unit-udt.cpp b/tests/src/unit-udt.cpp
index 7952442cf..2cecca3d9 100644
--- a/tests/src/unit-udt.cpp
+++ b/tests/src/unit-udt.cpp
@@ -14,7 +14,9 @@ DOCTEST_GCC_SUPPRESS_WARNING("-Wnoexcept")
 
 #include <nlohmann/json.hpp>
 using nlohmann::json;
-using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#ifdef JSON_TEST_NO_GLOBAL_UDLS
+    using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#endif
 
 #include <map>
 #include <memory>