Putting a Comma after However (i.e., a Transitional Phrase)
 
When a term like However, As a result, or Consequently starts a sentence, it should be followed by a comma. (These terms are called transitional phrases.)

A transitional phrase sits at the start of a sentence and acts like a bridge to an idea in the previous sentence. Common ones are However, Consequently, Therefore, and As a result.

A transitional phrase is followed by a comma. A transitional phrase is never preceded by a comma.
 

Use a Comma after a Transitional Phrase (e.g., However,)

It is common for a sentence to start with an introduction that acts like a bridge between the last sentence and the new sentence. The introduction makes the transition between the two sentences smoother. (It is known as a transitional phrase.)

A transitional phrase always appears at the start of a sentence and is followed by a comma. For example:

  • Bruce Leonard spent 4 years in Japan studying Kung Fu. As a result, he is often able to predict moves by Japanese opponents.
  • (As a result is a transitional phrase. It acts like a bridge between the previous sentence and the new one. It is followed by a comma.)

  • Mark was separated from his twin sister when they were both one. Of course, it was often said that the girl next door looked a little like him, but no one had any reason to think they might be related.
  • (Of course is a transitional phrase.)

  • In summary, 70% of you will fail the course.

  • Consequently, I cannot tolerate sloppy work.

  • Consequently, his teaching techniques are flawed.

  • However, Bruce was unaware that the shark was only making its first pass.

Beware Putting a Comma before However

The word However is a very common transitional phrase. It is just like the other transitional phrases, but it deserves a special mention because writers often mistakenly precede it with a comma (as opposed to using it to start a sentence).

Examples:

  • I don't like cake. However, I love scones.
  • I don't like cake, however, I love scones.
When However is being used as a transitional phrase, preceding it with a comma is a mistake. It is called a run-on sentence.

On occasion, it might be appropriate to precede however with a semicolon. For example:

  • I don't like cake; however, I love scones.
 
YOU CAN USE SEMICOLONS

On occasion, you may wish to use a semicolon before a transitional phrase to make the transition between sentences even more seamless. For example:

  • I cannot come on Tuesday. However, Peter will be there.
  • I cannot come on Tuesday; however, Peter will be there.
Read more about using semicolons before transitional phrases.

DON'T OVERUSE SEMICOLONS

Transitional phrases are useful because they keep ideas flowing between sentences. Most of them should be preceded by full stops / ( periods.

If you use semicolons before all your transitional phrases, you will annoy your readers. Semicolons are great in moderation. Hold them back for when you want a really smooth transition between two sentences.
 
 
NOT A COMMA

You cannot use a comma before a transitional phrase. For example:

  • I cannot come on Tuesday, however, Peter will be there.
This is a very common error – especially with however.

Read more about the error known as a run-on sentence.
 

See also:

Run-on errors with commas
Using semicolons before transitional phrases (e.g. however)
Using semicolons to extend a sentence
Commas after a sentence introductions
Commas after interjections (yes, no, indeed)
Commas before conjunctions (and, or, but)
Commas for parenthesis
Commas in lists
Commas with a long subject
Commas with numbers
Commas with quotation (speech) marks
Commas with the vocative case
Commas with Dear, Hello, and Hi
List of easily confused words