Jessica Simpson may swear by her snake semen drink that she says helps her sing … but at least one expert tells TMZ the truth is even more of a sticky situation.

Here’s the deal … Jessica recently raved to us about her secret specialty Chinese herb cocktail that she says has legit serpent swimmers in it — all at the recommendation of her vocal coach.

Zoologist and “Reptile Royalty” star Jay Brewer tells TMZ … it’s highly unlikely Jessica’s concoction has any actual snake sperm in it … ’cause frankly, it’s not that easy to come by (no pun)!

It seems extracting the fluid is super tough … and even in the largest snakes around the globe, the amount is usually minuscule.

To be clear, Jay ain’t calling Jessica a liar … he just suspects she’s been duped — and the secret ingredient she’s actually consuming is snake oil, AKA nothing special.

That said, Jay said anything is possible … and if JS is really drinking snake semen, the possible benefits would probably be a result of stem cells.

Of course, she isn’t the first celeb to turn to animal sperm for personal gain … Kim Kardashian and Jennifer Aniston both swear by salmon sperm as a route to achieving younger-looking skin.

The whole situation reminds Jay of a tradition done in Southeast Asia where locals put the gall bladder of a snake in their booze and drink it — to bring them luck.

Whether it be a gall bladder for luck or semen for the vocals … Jay tells us it’s all BS in his book … and just a mistreatment of animals.