Can I say/write
“It’s advertised” instead of “It was advertised”
It’s is a contraction that can be used for
it is or
it has only.
For instance, the sentence, “It is advertised as a slimming drug,” can be written as, “It’s advertised as a slimming drug.”
Similarly, the sentence, “It has been advertised as slimming drug,” can also be written as, “It’s been advertised as a slimming drug.”
You cannot use this contraction for “it was”.
Hope this helps.
PS: If you need a dictionary reference, here it is: http://dictionary.reference.com/browse/it%27s