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Jimmy Fallon and the "Dolittle" star donned their best '90s looks to bring to life a sunny parody of those creepy prescription medication ads.

One of the biggest challenges for anyone is trying to cold read something without screwing it up, or breaking character ... just as Bill Hader, who struggled every time Stefon appeared on "Saturday Night Live."

Well, Jimmy Fallon brought that challenge to Robert Downey Jr. on "The Tonight Show" Wednesday night with a parody ad for an allergy medication and we have to give props where they're due. Robert did a fantastic job (mostly) of keeping it together as he and Fallon took turns reading some of the fake ad's ridiculous lines.

Her certainly did better than Fallon, old-school "SNL" fans know that he laughed through just about every sketch he appeared in throughout his run on the venerable late-night sketch show.

We really thought they'd get into "common side effects" for the crazy directions they could have gone there, but instead they treated it like they were satisfied new patients taking this medication and just talking about it. And other random, sometimes unrelated stuff.

"If your erection lasts more than four hours, that has nothing to do with Zynerma," Robert said with a serious look on his face, before adding, "But congratulations!

"Did you know you can make your own homemade Tylenol?" Jimmy asked. "All you need is Skittles and vodka."

Before each line, they had their backs to the camera, so when they turned they'd be seeing their lines for the first time. And yet, Robert was still able to emote properly as he said, "Here’s a list of everything I’m allergic to: love."

Apparently the contents of the sketch wasn't the only thing the guys weren't made aware of, as they both thought it was over before they were even halfway done. And things just kept getting more and more ridiculous.

Finally, the two of them had to share an exchange: "I lived in the back of a Walgreens for three years because my arm was stuck in the blood pressure machine."

This was a sordid tale that somehow became fodder for an episode of "My 600-Pound Life" with hints of "To Catch a Predator." It was -- appropriately -- weird.

We're not sure if it was more ridiculous what they were saying or their attempt to capture the look and styles of people in the '90s. Okay, that part was scarily accurate. Let's just say grunge was not represented in medication commercials.

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