That was immediately apparent from your first sentence:
There is a common knowledge that many artists were using drugs
While not strictly ungrammatical, this sounds extremely awkward and “foreign-accented”. Replace with:
It is common knowledge that many artists have used drugs
(Note the absence of the indefinite article; it is very rare to speak of “a knowledge”. “Knowledge” is an unquantifiable noun, like “stuff”.)
or
It is commonly known that many artists have used drugs
(Also note that in replacing the inappropriate verb tense “were using” with “have used”, I have also changed the meaning slightly to reflect the fact that there are still artists in existence today. If you had been speaking of a group of people that is no longer extant, you would have needed to say, for example,
It is commonly known that many ancient Romans used drugs. (Not “have used”.)
That was immediately apparent from your first sentence:
While not strictly ungrammatical, this sounds extremely awkward and “foreign-accented”. Replace with:
(Note the absence of the indefinite article; it is very rare to speak of “a knowledge”. “Knowledge” is an unquantifiable noun, like “stuff”.)
or
(Also note that in replacing the inappropriate verb tense “were using” with “have used”, I have also changed the meaning slightly to reflect the fact that there are still artists in existence today. If you had been speaking of a group of people that is no longer extant, you would have needed to say, for example,
Thanks for the detailed explanation. I changed the sentence.