diff --git a/src/units.jl b/src/units.jl
index 7c016d67..56de32bf 100644
--- a/src/units.jl
+++ b/src/units.jl
@@ -6,24 +6,35 @@ import ..DEFAULT_QUANTITY_TYPE
 
 @assert DEFAULT_VALUE_TYPE == Float64 "`units.jl` must be updated to support a different default value type."
 
-const _UNIT_SYMBOLS = Symbol[]
-const _UNIT_VALUES = DEFAULT_QUANTITY_TYPE[]
+const UNIT_SYMBOLS = Symbol[]
+const UNIT_VALUES = DEFAULT_QUANTITY_TYPE[]
 
 macro register_unit(name, value)
     return esc(_register_unit(name, value))
 end
 
+macro _lazy_register_unit(name, value)
+    return esc(_register_unit(name, value; lazy = true))
+end
+
 macro add_prefixes(base_unit, prefixes)
     @assert prefixes.head == :tuple
     return esc(_add_prefixes(base_unit, prefixes.args, _register_unit))
 end
 
-function _register_unit(name::Symbol, value)
-    s = string(name)
+const UNIT_MAPPING = Dict{Symbol,Int}()
+function update_unit_mapping(name, value, unit_mapping::Dict{Symbol, Int} = UNIT_MAPPING)
+    unit_mapping[name] = length(unit_mapping) + 1
+end
+
+function _register_unit(name::Symbol, value; lazy = false)
+    name_symbol = Meta.quot(name)
     return quote
+        haskey($UNIT_MAPPING, $name_symbol) && throw("Unit $name_symbol already exists.")
         const $name = $value
-        push!(_UNIT_SYMBOLS, Symbol($s))
-        push!(_UNIT_VALUES, $name)
+        push!($UNIT_SYMBOLS, $name_symbol)
+        push!($UNIT_VALUES, $name)
+        !$lazy && $update_unit_mapping($name_symbol, $value)
     end
 end
 
@@ -42,13 +53,13 @@ function _add_prefixes(base_unit::Symbol, prefixes, register_function)
 end
 
 # SI base units
-@register_unit m DEFAULT_QUANTITY_TYPE(1.0, length=1)
-@register_unit g DEFAULT_QUANTITY_TYPE(1e-3, mass=1)
-@register_unit s DEFAULT_QUANTITY_TYPE(1.0, time=1)
-@register_unit A DEFAULT_QUANTITY_TYPE(1.0, current=1)
-@register_unit K DEFAULT_QUANTITY_TYPE(1.0, temperature=1)
-@register_unit cd DEFAULT_QUANTITY_TYPE(1.0, luminosity=1)
-@register_unit mol DEFAULT_QUANTITY_TYPE(1.0, amount=1)
+@_lazy_register_unit m DEFAULT_QUANTITY_TYPE(1.0, length = 1)
+@_lazy_register_unit g DEFAULT_QUANTITY_TYPE(1e-3, mass = 1)
+@_lazy_register_unit s DEFAULT_QUANTITY_TYPE(1.0, time = 1)
+@_lazy_register_unit A DEFAULT_QUANTITY_TYPE(1.0, current = 1)
+@_lazy_register_unit K DEFAULT_QUANTITY_TYPE(1.0, temperature = 1)
+@_lazy_register_unit cd DEFAULT_QUANTITY_TYPE(1.0, luminosity = 1)
+@_lazy_register_unit mol DEFAULT_QUANTITY_TYPE(1.0, amount = 1)
 
 @add_prefixes m (f, p, n, μ, u, c, d, m, k, M, G)
 @add_prefixes g (p, n, μ, u, m, k)
@@ -88,17 +99,17 @@ end
 )
 
 # SI derived units
-@register_unit Hz inv(s)
-@register_unit N kg * m / s^2
-@register_unit Pa N / m^2
-@register_unit J N * m
-@register_unit W J / s
-@register_unit C A * s
-@register_unit V W / A
-@register_unit F C / V
-@register_unit Ω V / A
-@register_unit ohm Ω
-@register_unit T N / (A * m)
+@_lazy_register_unit Hz inv(s)
+@_lazy_register_unit N kg * m / s^2
+@_lazy_register_unit Pa N / m^2
+@_lazy_register_unit J N * m
+@_lazy_register_unit W J / s
+@_lazy_register_unit C A * s
+@_lazy_register_unit V W / A
+@_lazy_register_unit F C / V
+@_lazy_register_unit Ω V / A
+@_lazy_register_unit ohm Ω
+@_lazy_register_unit T N / (A * m)
 
 @add_prefixes Hz (n, μ, u, m, k, M, G)
 @add_prefixes N ()
@@ -156,17 +167,17 @@ end
 
 # Common assorted units
 ## Time
-@register_unit min 60 * s
-@register_unit minute min
-@register_unit h 60 * min
-@register_unit hr h
-@register_unit day 24 * h
-@register_unit d day
-@register_unit wk 7 * day
-@register_unit yr 365.25 * day
-@register_unit inch 2.54 * cm
-@register_unit ft 12 * inch
-@register_unit mi 5280 * ft
+@_lazy_register_unit min 60 * s
+@_lazy_register_unit minute min
+@_lazy_register_unit h 60 * min
+@_lazy_register_unit hr h
+@_lazy_register_unit day 24 * h
+@_lazy_register_unit d day
+@_lazy_register_unit wk 7 * day
+@_lazy_register_unit yr 365.25 * day
+@_lazy_register_unit inch 2.54 * cm
+@_lazy_register_unit ft 12 * inch
+@_lazy_register_unit mi 5280 * ft
 
 @add_prefixes min ()
 @add_prefixes minute ()
@@ -178,7 +189,7 @@ end
 @add_prefixes yr (k, M, G)
 
 ## Volume
-@register_unit L dm^3
+@_lazy_register_unit L dm^3
 
 @add_prefixes L (μ, u, m, c, d)
 
@@ -203,9 +214,7 @@ end
 # Do not wish to define physical constants, as the number of symbols might lead to ambiguity.
 # The user should define these instead.
 
-"""A tuple of all possible unit symbols."""
-const UNIT_SYMBOLS = Tuple(_UNIT_SYMBOLS)
-const UNIT_VALUES = Tuple(_UNIT_VALUES)
-const UNIT_MAPPING = NamedTuple([s => i for (i, s) in enumerate(UNIT_SYMBOLS)])
+# Update `UNIT_MAPPING` with all internally defined unit symbols.
+merge!(UNIT_MAPPING, Dict(UNIT_SYMBOLS .=> 1:lastindex(UNIT_SYMBOLS)))
 
 end
diff --git a/test/unittests.jl b/test/unittests.jl
index 57074e1a..4fc07dcc 100644
--- a/test/unittests.jl
+++ b/test/unittests.jl
@@ -1,16 +1,18 @@
 using DynamicQuantities
 using DynamicQuantities: FixedRational, NoDims, AbstractSymbolicDimensions
-using DynamicQuantities: DEFAULT_QUANTITY_TYPE, DEFAULT_DIM_BASE_TYPE, DEFAULT_DIM_TYPE, DEFAULT_VALUE_TYPE
+using DynamicQuantities:
+    DEFAULT_QUANTITY_TYPE, DEFAULT_DIM_BASE_TYPE, DEFAULT_DIM_TYPE, DEFAULT_VALUE_TYPE
 using DynamicQuantities: array_type, value_type, dim_type, quantity_type
 using DynamicQuantities: GenericQuantity, with_type_parameters, constructorof
 using DynamicQuantities: promote_quantity_on_quantity, promote_quantity_on_value
+using DynamicQuantities.Units: @register_unit, UNIT_VALUES, UNIT_MAPPING
 using DynamicQuantities: map_dimensions
 using Ratios: SimpleRatio
 using SaferIntegers: SafeInt16
 using StaticArrays: SArray, MArray
 using LinearAlgebra: norm
 using Test
-
+#=
 function record_show(s, f=show)
     io = IOBuffer()
     f(io, s)
@@ -1721,7 +1723,7 @@ end
         ) isa SymbolicDimensions{Int32}
 
     @test copy(km) == km
-    
+
     # Any operation should immediately convert it:
     @test km ^ -1 isa Quantity{T,DynamicQuantities.SymbolicDimensions{R}} where {T,R}
 
@@ -1842,3 +1844,23 @@ end
     y = Quantity(2.0im, mass=1)
     @test_throws DimensionError x^y
 end
+=#
+
+# `@testset` rewrites the test block with a `let...end`, resulting in an invalid
+# local `const` (ref: src/units.jl:26). To avoid it, register units outside the
+# test block.
+map_count_before_registering = length(UNIT_MAPPING)
+@register_unit MyV u"V"
+@register_unit MySV us"V"
+
+@testset "Register Unit" begin
+    @test MyV === u"V"
+    @test MyV == us"V"
+    @test MySV == us"V"
+
+    @test length(UNIT_MAPPING) == map_count_before_registering + 2
+
+    for my_unit in (MySV, MyV)
+        @test my_unit in UNIT_VALUES
+    end
+end