The comedian is being hounded by allegations he's using one of the weight loss drugs -- like Ozempic or Wegovy -- which are super popular these days, especially among celebs looking to shed pounds.

For his part, Lil Rel says his new look is all from diet and exercise -- specifically, he's chalking it up to 5 years of physical, mental and spiritual work on himself.

In a social media post flaunting his toned body, Lil Rel says he's proud of himself for looking and feeling his best for the first time in his life.

He says his secret is simple ... "To stop drinking, eating better, working out almost every single day, writing, building a closer relationship with God, therapy and honestly accountability has taken my life to the next level of pure happiness."

But, scores of folks online aren't so easily convinced, which means his critics are sticking to their Ozempic guns.

Among the pointed replies to LR ... "He's lying. He's not active. You have no muscle definition to show you've been actively working out, just frail and massless." Ouch.

Lil Rel added a video of himself shadowboxing while holding some dumbbells -- and, still, one user says, "You ain't fooling nobody."