Barcelona defender De Boer, the Dutch captain, and Juventus midfielder Edgar Davids face a possible two-year ban after testing positive for the banned substance nandrolone.
Both players deny knowingly taking the drug, and although there are fears other Dutch internationals could also have illegal levels, Westerveld was quoted on Liverpool's website as saying he was unconcerned.
"I have never taken nandrolone and I was selected for a random drug test after our (Liverpool's) game against Porto on 15 March," he said.
"That is a significant date because I believe it was the night that Frank De Boer tested positive.
"I want to make it perfectly clear that I am 100% sure that Frank and Edgar Davids have never taken any illegal substance. I cannot stress that enough. They both have my full support."
De Boer tested positive after Barcelona's Uefa Cup quarter-final against Celta Vigo on 15 March, while Davids's "non-negative" first test was on 4 March after a Serie A match against Udinese.
Davids is also waiting for the results of a second test.
Fritz Kessel, the Dutch team doctor, maintains that all of the squad could risk a positive test and blames supplements given to the players while on international duty.
"The players who have played for Holland this season must be checked as soon as possible because they do not know what substances are in their body at the moment and whether or not they will be tested positive," he said.
De Boer and Davids both played in the friendly against Turkey on 28 February at the Amsterdam Arena.