diff --git a/lib/faraday.rb b/lib/faraday.rb
index 3e99aeca4..5a192bef3 100644
--- a/lib/faraday.rb
+++ b/lib/faraday.rb
@@ -153,7 +153,7 @@ def self.default_connection_options=(options)
     @default_connection_options = ConnectionOptions.from(options)
   end
 
-  unless const_defined? :Timer
+  unless defined?(::Faraday::Timer)
     require 'timeout'
     Timer = Timeout
   end
diff --git a/lib/faraday/adapter/em_http.rb b/lib/faraday/adapter/em_http.rb
index acadc35ff..156da5a52 100644
--- a/lib/faraday/adapter/em_http.rb
+++ b/lib/faraday/adapter/em_http.rb
@@ -142,7 +142,8 @@ def perform_request(env)
 
         raise Faraday::ConnectionFailed, e
       rescue StandardError => e
-        if defined?(OpenSSL) && e.is_a?(OpenSSL::SSL::SSLError)
+        if defined?(::OpenSSL::SSL::SSLError) && \
+           e.is_a?(::OpenSSL::SSL::SSLError)
           raise Faraday::SSLError, e
         end
 
diff --git a/lib/faraday/adapter/httpclient.rb b/lib/faraday/adapter/httpclient.rb
index 539c15aaa..cc563f13e 100644
--- a/lib/faraday/adapter/httpclient.rb
+++ b/lib/faraday/adapter/httpclient.rb
@@ -66,7 +66,8 @@ def call(env)
       rescue Errno::EADDRNOTAVAIL, Errno::ECONNREFUSED, IOError, SocketError
         raise Faraday::ConnectionFailed, $ERROR_INFO
       rescue StandardError => e
-        if defined?(OpenSSL) && e.is_a?(OpenSSL::SSL::SSLError)
+        if defined?(::OpenSSL::SSL::SSLError) && \
+           e.is_a?(::OpenSSL::SSL::SSLError)
           raise Faraday::SSLError, e
         end
 
diff --git a/lib/faraday/adapter/net_http.rb b/lib/faraday/adapter/net_http.rb
index 191568dd4..48f673f39 100644
--- a/lib/faraday/adapter/net_http.rb
+++ b/lib/faraday/adapter/net_http.rb
@@ -30,8 +30,10 @@ class NetHttp < Faraday::Adapter
         Zlib::GzipFile::Error
       ]
 
-      exceptions << OpenSSL::SSL::SSLError if defined?(OpenSSL)
-      exceptions << Net::OpenTimeout if defined?(Net::OpenTimeout)
+      if defined?(::OpenSSL::SSL::SSLError)
+        exceptions << ::OpenSSL::SSL::SSLError
+      end
+      exceptions << ::Net::OpenTimeout if defined?(::Net::OpenTimeout)
 
       NET_HTTP_EXCEPTIONS = exceptions.freeze
 
diff --git a/lib/faraday/request/authorization.rb b/lib/faraday/request/authorization.rb
index 5797016d7..f9b56c871 100644
--- a/lib/faraday/request/authorization.rb
+++ b/lib/faraday/request/authorization.rb
@@ -4,7 +4,9 @@ module Faraday
   class Request
     # Request middleware for the Authorization HTTP header
     class Authorization < Faraday::Middleware
-      KEY = 'Authorization' unless defined? KEY
+      unless defined?(::Faraday::Request::Authorization::KEY)
+        KEY = 'Authorization'
+      end
 
       # @param type [String, Symbol]
       # @param token [String, Symbol, Hash]
diff --git a/lib/faraday/request/multipart.rb b/lib/faraday/request/multipart.rb
index 399c434f4..9046aef66 100644
--- a/lib/faraday/request/multipart.rb
+++ b/lib/faraday/request/multipart.rb
@@ -8,7 +8,7 @@ class Request
     # Middleware for supporting multi-part requests.
     class Multipart < UrlEncoded
       self.mime_type = 'multipart/form-data'
-      unless defined? DEFAULT_BOUNDARY_PREFIX
+      unless defined?(::Faraday::Request::Multipart::DEFAULT_BOUNDARY_PREFIX)
         DEFAULT_BOUNDARY_PREFIX = '-----------RubyMultipartPost'
       end
 
diff --git a/lib/faraday/request/url_encoded.rb b/lib/faraday/request/url_encoded.rb
index 7534b267d..00cc973dd 100644
--- a/lib/faraday/request/url_encoded.rb
+++ b/lib/faraday/request/url_encoded.rb
@@ -4,7 +4,9 @@ module Faraday
   class Request
     # Middleware for supporting urlencoded requests.
     class UrlEncoded < Faraday::Middleware
-      CONTENT_TYPE = 'Content-Type' unless defined? CONTENT_TYPE
+      unless defined?(::Faraday::Request::UrlEncoded::CONTENT_TYPE)
+        CONTENT_TYPE = 'Content-Type'
+      end
 
       class << self
         attr_accessor :mime_type