diff --git a/Project.toml b/Project.toml
index 74aa3da..e6bdb26 100644
--- a/Project.toml
+++ b/Project.toml
@@ -7,6 +7,12 @@ Calculus = "49dc2e85-a5d0-5ad3-a950-438e2897f1b9"
 NaNMath = "77ba4419-2d1f-58cd-9bb1-8ffee604a2e3"
 SpecialFunctions = "276daf66-3868-5448-9aa4-cd146d93841b"
 
+[weakdeps]
+SpecialFunctions = "276daf66-3868-5448-9aa4-cd146d93841b"
+
+[extensions]
+DualNumbersSpecialFunctionsExt = "SpecialFunctions"
+
 [compat]
 Calculus = "0.5"
 NaNMath = "0.3, 1"
@@ -15,7 +21,8 @@ julia = "0.7, 1.0"
 
 [extras]
 LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"
+SpecialFunctions = "276daf66-3868-5448-9aa4-cd146d93841b"
 Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 
 [targets]
-test = ["LinearAlgebra", "Test"]
+test = ["LinearAlgebra", "Test", "SpecialFunctions"]
diff --git a/ext/DualNumbersSpecialFunctionsExt.jl b/ext/DualNumbersSpecialFunctionsExt.jl
new file mode 100644
index 0000000..750393b
--- /dev/null
+++ b/ext/DualNumbersSpecialFunctionsExt.jl
@@ -0,0 +1,29 @@
+module DualNumbersSpecialFunctionsExt
+
+using DualNumbers
+using DualNumbers: value, epsilon, to_nanmath
+using NaNMath
+using Calculus
+using SpecialFunctions
+
+for (funsym, expr) in Calculus.symbolic_derivatives_1arg()
+    if isdefined(SpecialFunctions, funsym) && !isdefined(Base, funsym)
+        @eval function SpecialFunctions.$(funsym)(z::Dual)
+            x = value(z)
+            xp = epsilon(z)
+            Dual($(funsym)(x),xp*$expr)
+        end
+    end
+    # extend corresponding NaNMath methods
+    if funsym in (:lgamma,)
+        funsym = Expr(:.,:NaNMath,Base.Meta.quot(funsym))
+        @eval function $(funsym)(z::Dual)
+            x = value(z)
+            xp = epsilon(z)
+            Dual($(funsym)(x),xp*$(to_nanmath(expr)))
+        end
+    end
+end
+
+
+end
diff --git a/src/DualNumbers.jl b/src/DualNumbers.jl
index 53e8c53..99642b4 100644
--- a/src/DualNumbers.jl
+++ b/src/DualNumbers.jl
@@ -1,6 +1,5 @@
 module DualNumbers
 
-using SpecialFunctions
 import NaNMath
 import Calculus
 
@@ -26,4 +25,8 @@ export
     ɛ,
     imɛ
 
+if !isdefined(Base, :get_extension)
+    include("../ext/DualNumbersSpecialFunctionsExt.jl")
+end
+
 end # module
diff --git a/src/dual.jl b/src/dual.jl
index 385c172..b84f531 100644
--- a/src/dual.jl
+++ b/src/dual.jl
@@ -303,31 +303,25 @@ to_nanmath(x) = x
 
 
 
-for (funsym, exp) in Calculus.symbolic_derivatives_1arg()
+for (funsym, expr) in Calculus.symbolic_derivatives_1arg()
     funsym == :exp && continue
     funsym == :abs2 && continue
     funsym == :inv && continue
-    if isdefined(SpecialFunctions, funsym)
-        @eval function SpecialFunctions.$(funsym)(z::Dual)
-            x = value(z)
-            xp = epsilon(z)
-            Dual($(funsym)(x),xp*$exp)
-        end
-    elseif isdefined(Base, funsym)
+    if isdefined(Base, funsym)
         @eval function Base.$(funsym)(z::Dual)
             x = value(z)
             xp = epsilon(z)
-            Dual($(funsym)(x),xp*$exp)
+            Dual($(funsym)(x),xp*$expr)
         end
     end
     # extend corresponding NaNMath methods
     if funsym in (:sin, :cos, :tan, :asin, :acos, :acosh, :atanh, :log, :log2, :log10,
-          :lgamma, :log1p)
+          :log1p)
         funsym = Expr(:.,:NaNMath,Base.Meta.quot(funsym))
         @eval function $(funsym)(z::Dual)
             x = value(z)
             xp = epsilon(z)
-            Dual($(funsym)(x),xp*$(to_nanmath(exp)))
+            Dual($(funsym)(x),xp*$(to_nanmath(expr)))
         end
     end
 end