Might need fleshing out slightly, but a concept I’ve always enjoyed is the ‘werewoman’ .
An online store is selling a sleep remedy, that when dropped on the tongue will give you ‘the perfect night’.
After taking it however, the user drifts in to a trance – before transforming in to a beautiful woman who goes out in to the night to bars and clubs.
After using it a few times, the effects start to happen of their own accord – first changing when the sun goes down, eventually happening at random during the day before eventually the change becomes permanent.
The transformation would be magical, and those taking it could try and find the woman that makes it in hopes of receiving a cure before it's too late.