diff --git a/hc/accounts/forms.py b/hc/accounts/forms.py
index 232a0fd3..aa68bb95 100644
--- a/hc/accounts/forms.py
+++ b/hc/accounts/forms.py
@@ -175,3 +175,5 @@ class TotpForm(forms.Form):
     def clean_code(self):
         if not self.totp.verify(self.cleaned_data["code"], valid_window=1):
             raise forms.ValidationError("The code you entered was incorrect.")
+
+        return self.cleaned_data["code"]
diff --git a/hc/accounts/tests/test_login_totp.py b/hc/accounts/tests/test_login_totp.py
index ed0ed767..afaa4521 100644
--- a/hc/accounts/tests/test_login_totp.py
+++ b/hc/accounts/tests/test_login_totp.py
@@ -84,3 +84,14 @@ class LoginTotpTestCase(BaseTestCase):
 
         r = self.client.post(self.url, {"code": "000000"})
         self.assertContains(r, "Too Many Requests")
+
+    @patch("hc.accounts.views.pyotp.totp.TOTP")
+    def test_it_rejects_used_code(self, mock_TOTP):
+        mock_TOTP.return_value.verify.return_value = True
+
+        obj = TokenBucket(value=f"totpc-{self.alice.id}-000000")
+        obj.tokens = 0
+        obj.save()
+
+        r = self.client.post(self.url, {"code": "000000"})
+        self.assertContains(r, "Too Many Requests")
diff --git a/hc/accounts/views.py b/hc/accounts/views.py
index d173ce42..db22627d 100644
--- a/hc/accounts/views.py
+++ b/hc/accounts/views.py
@@ -848,11 +848,14 @@ def login_totp(request):
 
     totp = pyotp.totp.TOTP(user.profile.totp)
     if request.method == "POST":
-        if not TokenBucket.authorize_totp(user):
+        if not TokenBucket.authorize_totp_attempt(user):
             return render(request, "try_later.html")
 
         form = forms.TotpForm(totp, request.POST)
         if form.is_valid():
+            if not TokenBucket.authorize_totp_code(user, form.cleaned_data["code"]):
+                return render(request, "try_later.html")
+
             request.session.pop("2fa_user")
             auth_login(request, user, "hc.accounts.backends.EmailBackend")
             return _redirect_after_login(request)
diff --git a/hc/api/models.py b/hc/api/models.py
index bfbf3b8f..fb4e3535 100644
--- a/hc/api/models.py
+++ b/hc/api/models.py
@@ -985,9 +985,18 @@ class TokenBucket(models.Model):
         return TokenBucket.authorize(value, 10, 3600 * 24)
 
     @staticmethod
-    def authorize_totp(user):
+    def authorize_totp_attempt(user):
         value = "totp-%d" % user.id
 
-        # 96 attempts per 24 hours
+        # 96 attempts per user per 24 hours
         # (or, on average, one attempt per 15 minutes)
         return TokenBucket.authorize(value, 96, 3600 * 24)
+
+    @staticmethod
+    def authorize_totp_code(user, code):
+        value = "totpc-%d-%s" % (user.id, code)
+
+        # A code has a validity period of 3 * 30 = 90 seconds.
+        # During that period, allow the code to only be used once,
+        # so an eavesdropping attacker cannot reuse a code.
+        return TokenBucket.authorize(value, 1, 90)