|When a word like and, or, and but (called conjunctions) joins two standalone sentences, you should put a comma before it.|
Put a Comma before a Conjunction If It Joins Two Independent ClausesWords like and, or, and but are known as conjunctions. (There are other conjunctions, but these three are by far the most common.)
Conjunctions are often used in lists, and the ruling about when to use a comma before a conjunction in a list depends on whether you're following US or UK writing conventions (or whether you're an advocate of the Oxford Comma). This page is about conjunctions that are used to merge two "sentences" into one using a conjunction. This is a very common practice. When a conjunction is used in this way, it is usual to place a comma before it.
(This is two sentences merged into one with but – comma required.)
comma before and in "vitamin, and the skin..." (This is correct.)
No Comma Necessary for Short ClausesIt is also worth knowing this:
If the two "sentences" (known as independent clauses) are very short, it is acceptable – for style purposes – to omit the comma.
TOO MANY COMMAS? USE A SEMICOLON
When sentences containing commas are merged together using a conjunction, it is possible to use a semicolon instead of a comma. (See lesson Semicolons for more examples.)
However, this is quite an outdated practice in modern writing. Use it very sparingly — if at all.