Unfortunately, when you and doc rolled in the tray of vials, you got a dose of a couple other ursine formulas. You find your cock swelling up, larger and larger, as doc begins to shrink, or as you start to realize, you're growing.
Something else is happening to doc as well. "Oh," he cries. "Oh..." then reaches down to his cock which is shrinking and rearranging itself, a slit opening underneath it as he starts changing into a she-bear. "Oh my goodness," doc says through his--her--muzzle, "the formular I derived from the russian bear, the only good sample I could get was from a female!"
The doc continues to shrink, or really you to grow. "What's happening to me?" you ask.
Doc looks up at you. "You've apparently been injected with the Kodiak formula, the world's largest bear. What's more, it appears to be hybridizing. Oh my goodness, look at your marvelous cock."
And it is marvelous, and huge, and doc begins nuzzling it, licking and sucking on it, then attempt to get it inside his new bear vagina, but it's too big and won't fit.
Just then, the doc's receptionist, Goldie, walks in to see a twenty-foot tall anthro-Kodiak-grizzly hybrid trying to schtupp an anthro black-russian bear hybrid as the immortal line from the three bears is said, "This one is too big!"