With his shirt off, Drew unbuttoned his pants and stripped off his underwear. Lilia trembled with anticipation at the sight of his hard dick. Then she felt butterflies in her stomach. Soon her tail began to tingle.
"Drew!" she shouted, "Something's wrong!" Her orange tail began to split down the middle. "No! No! Not now!"
Lilia watched disappointedly as her beautiful tail fully split in two. Her fins shrank and reshaped themselves into feet. Last of all, her orange scales began to disappear. Soon Lilia was completely human from head to toe. She gazed down remorsefully at her new feet.
"Damn!" she cursed, "I was looking forward to sex as a mermaid!"
"Hey, at least you're alright!" said Drew.
"I guess you're right," said Lilia, "I wonder what happened, though. I haven't taken any more serum since the failed dose."
"I say its time we checked the label on the bottle," said Drew. Lilia nodded.
She found the bottle and inspected it. After a little scanning, she found this: "Constant usage may cause a resistance to build up in the user. This usually manifests as a delayed reaction of up to 15 minutes. Afterwards, the serum should take effect as normal."
"So that's it!" said Lilia.
"At least now we know you're no stuck as a mermaid forever," said Drew, "And I guess mersex is back on. Just don't take your serum dose tomorrow."
"I can hardly wait!" Lilia giggled. She and Drew kissed passionately.