diff --git a/zerver/lib/actions.py b/zerver/lib/actions.py
index 8afed8837d..2006779fd3 100644
--- a/zerver/lib/actions.py
+++ b/zerver/lib/actions.py
@@ -25,6 +25,7 @@ from zerver.models import Realm, RealmEmoji, Stream, UserProfile, UserActivity,
     ScheduledJob, realm_filters_for_domain, get_owned_bot_dicts, \
     get_old_unclaimed_attachments, get_cross_realm_users
 
+from zerver.lib.alert_words import alert_words_in_realm
 from zerver.lib.avatar import get_avatar_url, avatar_url
 
 from django.db import transaction, IntegrityError
@@ -629,8 +630,12 @@ def do_send_message(message, rendered_content = None, no_log = False, stream = N
 
 def render_incoming_message(message, content):
     # type: (Message, text_type) -> text_type
+    realm_alert_words = alert_words_in_realm(message.get_realm())
     try:
-        rendered_content = message.render_markdown(content)
+        rendered_content = message.render_markdown(
+            content=content,
+            realm_alert_words=realm_alert_words,
+        )
     except BugdownRenderingException:
         raise JsonableError(_('Unable to render message'))
     return rendered_content
diff --git a/zerver/lib/bugdown/__init__.py b/zerver/lib/bugdown/__init__.py
index 166e896f66..f9dba8259b 100644
--- a/zerver/lib/bugdown/__init__.py
+++ b/zerver/lib/bugdown/__init__.py
@@ -887,23 +887,28 @@ class AlertWordsNotificationProcessor(markdown.preprocessors.Preprocessor):
     def run(self, lines):
         # type: (Iterable[text_type]) -> Iterable[text_type]
         if current_message and db_data is not None:
-            # We check for a user's custom notifications here, as we want
-            # to check for plaintext words that depend on the recipient.
-            realm_words = db_data['realm_alert_words']
+            # We check for custom alert words here, the set of which are
+            # dependent on which users may see this message.
+            #
+            # Our caller passes in the list of possible_words.  We
+            # don't do any special rendering; we just append the alert words
+            # we find to the set current_message.alert_words.
+
+            realm_words = db_data['possible_words']
+
             content = '\n'.join(lines).lower()
 
             allowed_before_punctuation = "|".join([r'\s', '^', r'[\(\".,\';\[\*`>]'])
             allowed_after_punctuation = "|".join([r'\s', '$', r'[\)\"\?:.,\';\]!\*`]'])
 
-            for user_id, words in six.iteritems(realm_words):
-                for word in words:
-                    escaped = re.escape(word.lower())
-                    match_re = re.compile(u'(?:%s)%s(?:%s)' %
-                                            (allowed_before_punctuation,
-                                             escaped,
-                                             allowed_after_punctuation))
-                    if re.search(match_re, content):
-                        current_message.user_ids_with_alert_words.add(user_id)
+            for word in realm_words:
+                escaped = re.escape(word.lower())
+                match_re = re.compile(u'(?:%s)%s(?:%s)' %
+                                        (allowed_before_punctuation,
+                                         escaped,
+                                         allowed_after_punctuation))
+                if re.search(match_re, content):
+                    current_message.alert_words.add(word)
 
         return lines
 
@@ -1132,8 +1137,8 @@ def log_bugdown_error(msg):
     could cause an infinite exception loop."""
     logging.getLogger('').error(msg)
 
-def do_convert(md, realm_domain=None, message=None):
-    # type: (markdown.Markdown, Optional[text_type], Optional[Message]) -> Optional[text_type]
+def do_convert(md, realm_domain=None, message=None, possible_words=None):
+    # type: (markdown.Markdown, Optional[text_type], Optional[Message], Optional[Set[text_type]]) -> Optional[text_type]
     """Convert Markdown to HTML, with Zulip-specific settings and hacks."""
     from zerver.models import get_active_user_dicts_in_realm, UserProfile
 
@@ -1155,7 +1160,10 @@ def do_convert(md, realm_domain=None, message=None):
     if message:
         realm_users = get_active_user_dicts_in_realm(message.get_realm())
 
-        db_data = {'realm_alert_words': alert_words.alert_words_in_realm(message.get_realm()),
+        if possible_words is None:
+            possible_words = set() # Set[text_type]
+
+        db_data = {'possible_words':    possible_words,
                    'full_names':        dict((user['full_name'].lower(), user) for user in realm_users),
                    'short_names':       dict((user['short_name'].lower(), user) for user in realm_users),
                    'emoji':             message.get_realm().get_emoji()}
@@ -1210,9 +1218,9 @@ def bugdown_stats_finish():
     bugdown_total_requests += 1
     bugdown_total_time += (time.time() - bugdown_time_start)
 
-def convert(md, realm_domain=None, message=None):
-    # type: (markdown.Markdown, Optional[text_type], Optional[Message]) -> Optional[text_type]
+def convert(md, realm_domain=None, message=None, possible_words=None):
+    # type: (markdown.Markdown, Optional[text_type], Optional[Message], Optional[Set[text_type]]) -> Optional[text_type]
     bugdown_stats_start()
-    ret = do_convert(md, realm_domain, message)
+    ret = do_convert(md, realm_domain, message, possible_words)
     bugdown_stats_finish()
     return ret
diff --git a/zerver/models.py b/zerver/models.py
index 1bc51bac6e..d7c0f328cd 100644
--- a/zerver/models.py
+++ b/zerver/models.py
@@ -1,5 +1,5 @@
 from __future__ import absolute_import
-from typing import Any, List, Set, Tuple, TypeVar, \
+from typing import Any, Dict, List, Set, Tuple, TypeVar, \
     Union, Optional, Sequence, AbstractSet
 from typing.re import Match
 from zerver.lib.str_utils import NonBinaryStr
@@ -46,6 +46,8 @@ import datetime
 # TODO: see #1379 to eliminate bugdown dependencies
 bugdown = None # type: Any
 
+RealmAlertWords = Dict[int, List[text_type]]
+
 MAX_SUBJECT_LENGTH = 60
 MAX_MESSAGE_LENGTH = 10000
 MAX_LANGUAGE_ID_LENGTH = 50 # type: int
@@ -807,8 +809,8 @@ class Message(ModelReprMixin, models.Model):
         # type: () -> Realm
         return self.sender.realm
 
-    def render_markdown(self, content, domain=None):
-        # type: (text_type, Optional[text_type]) -> text_type
+    def render_markdown(self, content, domain=None, realm_alert_words=None):
+        # type: (text_type, Optional[text_type], Optional[RealmAlertWords]) -> text_type
         """Return HTML for given markdown. Bugdown may add properties to the
         message object such as `mentions_user_ids` and `mentions_wildcard`.
         These are only on this Django object and are not saved in the
@@ -823,7 +825,7 @@ class Message(ModelReprMixin, models.Model):
         self.mentions_wildcard = False
         self.is_me_message = False
         self.mentions_user_ids = set() # type: Set[int]
-        self.user_ids_with_alert_words = set() # type: Set[int]
+        self.alert_words = set() # type: Set[text_type]
 
         if not domain:
             domain = self.sender.realm.domain
@@ -831,7 +833,21 @@ class Message(ModelReprMixin, models.Model):
             # Use slightly customized Markdown processor for content
             # delivered via zephyr_mirror
             domain = u"zephyr_mirror"
-        rendered_content = bugdown.convert(content, domain, self)
+
+        possible_words = set() # type: Set[text_type]
+        if realm_alert_words is not None:
+            for words in realm_alert_words.values():
+                possible_words.update(set(words))
+
+        # DO MAIN WORK HERE -- call bugdown to convert
+        rendered_content = bugdown.convert(content, domain, self, possible_words)
+
+        self.user_ids_with_alert_words = set() # type: Set[int]
+
+        if realm_alert_words is not None:
+            for user_id, words in realm_alert_words.items():
+                if set(words).intersection(self.alert_words):
+                    self.user_ids_with_alert_words.add(user_id)
 
         self.is_me_message = Message.is_status_message(content, rendered_content)
 
diff --git a/zerver/tests/test_bugdown.py b/zerver/tests/test_bugdown.py
index 572184488a..a3248133b9 100644
--- a/zerver/tests/test_bugdown.py
+++ b/zerver/tests/test_bugdown.py
@@ -11,6 +11,7 @@ from zerver.lib.actions import (
     do_set_alert_words,
     get_realm,
 )
+from zerver.lib.alert_words import alert_words_in_realm
 from zerver.lib.camo import get_camo_url
 from zerver.lib.request import (
     JsonableError,
@@ -417,14 +418,18 @@ class BugdownTest(TestCase):
         user_profile = get_user_profile_by_email("othello@zulip.com")
         do_set_alert_words(user_profile, ["ALERTWORD", "scaryword"])
         msg = Message(sender=user_profile, sending_client=get_client("test"))
+        realm_alert_words = alert_words_in_realm(user_profile.realm)
+
+        def render(msg, content):
+            return msg.render_markdown(content, realm_alert_words=realm_alert_words)
 
         content = "We have an ALERTWORD day today!"
-        self.assertEqual(msg.render_markdown(content), "<p>We have an ALERTWORD day today!</p>")
+        self.assertEqual(render(msg, content), "<p>We have an ALERTWORD day today!</p>")
         self.assertEqual(msg.user_ids_with_alert_words, set([user_profile.id]))
 
         msg = Message(sender=user_profile, sending_client=get_client("test"))
         content = "We have a NOTHINGWORD day today!"
-        self.assertEqual(msg.render_markdown(content), "<p>We have a NOTHINGWORD day today!</p>")
+        self.assertEqual(render(msg, content), "<p>We have a NOTHINGWORD day today!</p>")
         self.assertEqual(msg.user_ids_with_alert_words, set())
 
     def test_mention_wildcard(self):