diff --git a/src/FHIR/MedicationRequest.php b/src/FHIR/MedicationRequest.php
index 36e948b..8ed4fc7 100644
--- a/src/FHIR/MedicationRequest.php
+++ b/src/FHIR/MedicationRequest.php
@@ -288,7 +288,7 @@ public function setDispenseRequest($quantity, $unit, $iter = 0, $validityPeriodS
         $this->medication_request['dispenseRequest'] = $dispense_request;
     }
 
-    public function setSubstitution($allowed = false, $reason = 'G')
+    public function setSubstitution($allowed = true, $reason = 'G')
     {
         $substitution['allowedBoolean'] = $allowed;