diff --git a/include/boost/fixed_string/detail/fixed_string.hpp b/include/boost/fixed_string/detail/fixed_string.hpp
index f0dc2b2..3a0542e 100644
--- a/include/boost/fixed_string/detail/fixed_string.hpp
+++ b/include/boost/fixed_string/detail/fixed_string.hpp
@@ -126,7 +126,11 @@ CharT*
 raw_to_string(CharT* last, std::size_t size, Integer i)
 {
     boost::ignore_unused(size);
+#ifdef BOOST_FIXED_STRING_USE_BOOST
     BOOST_ASSERT(size >= max_digits(sizeof(Integer)));
+#else
+    assert(size >= max_digits(sizeof(Integer)));
+#endif
     return raw_to_string<CharT, Integer, Traits>(
         last, i, std::is_signed<Integer>{});
 }
diff --git a/include/boost/fixed_string/fixed_string.hpp b/include/boost/fixed_string/fixed_string.hpp
index d4f7eaa..fb3e988 100644
--- a/include/boost/fixed_string/fixed_string.hpp
+++ b/include/boost/fixed_string/fixed_string.hpp
@@ -916,7 +916,11 @@ public:
     void
     pop_back()
     {
+#ifdef BOOST_FIXED_STRING_USE_BOOST
         BOOST_ASSERT(n_ > 0);
+#else
+        assert(n > 0);
+#endif
         Traits::assign(s_[--n_], 0);
     }
 
diff --git a/include/boost/fixed_string/impl/fixed_string.hpp b/include/boost/fixed_string/impl/fixed_string.hpp
index 2e56f59..15c7862 100644
--- a/include/boost/fixed_string/impl/fixed_string.hpp
+++ b/include/boost/fixed_string/impl/fixed_string.hpp
@@ -795,7 +795,11 @@ to_fixed_string(Integer x)
 {
     using CharT = char;
     using Traits = std::char_traits<CharT>;
+#ifdef BOOST_FIXED_STRING_USE_BOOST
     BOOST_STATIC_ASSERT(std::is_integral<Integer>::value);
+#else
+    static_assert(std::is_integral<Integer>::value);
+#endif
     char buf[detail::max_digits(sizeof(Integer))];
     auto last = buf + sizeof(buf);
     auto it = detail::raw_to_string<