She'll build up a tolerance

"They'll stop working at all," Caitlin said. "Same as any medication. If you take too much you'll get a tolerance, before..."

Iris's eyes widened. Oh god, she couldn't be serious.

"E-even the one?" Iris said.

"You'll get an immunity," Caitlin said. "I'd be surprised if you weren't already feeling the effects. I made them especially to work in single doses; if you've been taking several doubles, the time they work for would have reduced, especially if the excess reacts with the dark matter in your system."

Iris squealed. She fidgeted, then squeaked again, hurrying a few steps so she could hide with her ass against a wall rather than bared to the cell.

"How long?" Iris said, cheeks burning.

Oh god, she couldn't even escape it like that. She'd have to actually care that so many people had seen her in her birthday suit, seen her shamelessly parading around the city.

She swallowed. Step one, change her blog picture and pray no one had clicked it yet, despite how much time she'd spent locked up and how much of a popular topic it was. Iris squealed. Step two, oh god she didn't even know, move to another country?!

"I don't know," Caitlin said. "They're made to be metabolised fast, but with your mutation, if there are any remnants of the second in your system trying to trick it, your immune system'll work especially hard to try and tackle it. It could happen with your next double dose, it could take weeks, but it will happen."

Iris whimpered.

A few seconds later and someone returned with her bag; Caitlin took it for her, letting Iris keep her arms over herself.

"So, um, that coat?" Iris said. "Or anything. I..."

"How long since you took your last pill?" Caitlin said.

"What? I don't- I, a couple of hours ago?" Iris said.

"You might not want it now, then," Caitlin said. "An average pill lasts three hours, approximately. but if you've started to get a tolerance you might not be able to get far before it-"

"I'm not waiting an hour stark naked!" Iris whispered frantically, not wanting to raise her voice and attract attention. She squirmed, her arms feeling inadequate.

"You could risk it," Caitlin said, slowly.

Iris glanced at the bag. Or she could forget Caitlin's warnings and take a second pill, get home and not care, and just hope that didn't leave her unable to ever get dressed again.