Trisodium citrate dihydrate

A delightful tale, well written - and a nice 'twist' on the old 'snowbound at Christmas' theme! I gave this one a solid '5'.
To the commenter (Sex4lf57?) who said that Viagra doesn't work this way, I'll toss at least a cautionary comment or two.
Viagra is derived from a medication (sildenafil citrate) that was originally designed for cardiac patients. To prevent stress on the heart (that can cause a heart attack), the chemical causes the capillaries in the body to open as wide as possible, allowing the blood to flow freely with less pressure being applied by the heart.
As such, it also opens the capillaries in the corpus cavernosum of the penis, which leads to an erection. Now, there's a 'valve' that limits blood flow to the penis, so as to allow urinary function, without urine being able to seep into the passages where sperm travels. When a guy urinates, the semen passages are cut off - and when he is erect, the urinary tract is cut off, because the urethra (tube in the penis) does double duty.
When there is sufficient erotic stimulus to cause that 'valve' to switch the seminal tract to 'open', it also opens the sphincters that control blood flow to the penis, so that the capillaries can inflate completely.
During the years (nearly 60) that sildenafil citrate has been in use in cardiac patients (under another drug-name), it was noticed that when a patient was taking the drug, he was more susceptible (at an older age) to more-solid erections. Gradually, the doctors prescribing the drug got 'field reports' from their patients, and forwarded them to the company that makes the drug. Its use as an ED drug is secondary, and took a long time for FDA approval to market such a drug to non-cardiac patients.
When the person taking the drug is in their late teens or early 20's, and has a quite healthy circulatory system - and is around a near-continual erotic stimulus, as is the main character in this story - the effect CAN be a near-perpetual erection. Even the attempts to diminish it by masturbation won't necessarily cause it to go down, when there is a hotly-desired woman present and making subtle erotic comments to the male.

Slut Laura is on her knees sucking cock as Mike returns from the bedroom where he'd placed Phillip's purple luggage bag.
Phillip's smug suggestion that Mike take over for Laura was the moment Mike had waited for, had prepared for, had patiently endured several weeks of torment for. When Laura ordered Mike to, Get your pansy ass over here he sighed, walked slowly up and between Phillip's legs, humbly handed her a goblet of her favorite wine and knelt down in front of Phillip's slime-covered cock.
Laura took a healthy swallow of Chardonnay while smiling knowingly at Phillip with her sparkling eyes. She sat close to her once and future Dom and they both sneered as Mike slowly unbuttoned and spread Phillip's shirt apart to sensually stroke his chest before undoing the belt and pulling Phillip's pants apart and down far enough to completely expose most of his crotch and balls.
Mike reverently grasped Phillip's throbbing bull cock and leaned in while opening his mouth in the classic O shape. Just before the bulging helmet touched his lips, he paused, turned his eyes upward towards his wife and smiled, then looked at the Alpha male and said, Phillip, you've come into our lives and exposed my wife and I for who we really are. I thank you for that.
Phillip chuckled and he and Laura waited expectantly for Mike's ultimate capitulation. It was at that moment that Mike jammed the head of a 3800kV compact stun gun he'd palmed directly into Phillip's ballsac with a sickening crunch. Phillip's body arched and went rigid and his eyes rolled back in their sockets as the clicking electrical discharge worked its magic. Laura pulled backwards with a look of passive confusion as the effects of the drugged wine hit her system, then slumped over and rolled off the couch with a thud. Mike ground the stun gun into Phillip's groin and kept pressing the 5-second trigger a total of 5 times before withdrawing and standing up.
Mike's trap had sprung. He wasn't a wimp, he wasn't a cuck, he'd been sickened by the revelation of his wife's history, her perversions and worst of all, her rapid transformation into a manipulative slut in her own right. He'd spent much of the past two weeks thoroughly researching and accumulating a laundry list of supplies.
Reaching behind the couch he grabbed a small bag and dumped the contents onto the floor. Picking up 2 sets of steel handcuffs and an angle-iron spreader, he positioned the unconscious pair side by side on the floor, quickly stripped them and then lifted Laura atop Phillip so they were almost face to face. He draped Phillip's arms around his wife's back and cuffed them, then looped his wife's over and under Phillips embrace and around his back and cuffed her wrists so the pair were hopelessly entangled. He attached the spreader bar to their four ankles and inserted and tightly secured two penis gags in their slack mouths.
Now came the part he relished most. Taking a large syringe filled with a potent cocktail of female hormones and chemicals (including a 50-percent solution of calcium chloride dihydrate), Mike calmly injected the concoction into Phillip's balls and his cock’s spongy corpus cavernosum.
Mike sat in his favorite chair, exhaled and patiently awaited for Phillip to revive. He focused on slipping a rubber ring on an elastrator a trusted friend had borrowed from a nearby farm. It was Mike's turn to smugly stare into Phillip's eyes when they fluttered open, enjoying seeing the disorientation quickly replaced by dawning panic. Between the gut-wrenching pain in his crotch, Laura's dead weight and the spreader bar, Phillip could barely move; the penis gag limited his ability to communicate to huffing, puffing and fitful spluttering.
Mike patted Phillip on his head and whispered, Now, here's how it's going to play out, you prick. I'm going to use this device to slip this very tight rubber ring around that ball sack I enjoyed crushing and shocking earlier. It will cut off blood circulation to your testicles. Within a few hours, they'll be about as useful as TITS on a BULL. LOL There's a pun in there but you'll only figure it out later.”
“Laura will regain consciousness in about two hours. Not likely you'll be able to escape but even if you do, the damage to your family jewels is well underway. If you go to the police, they'll learn that you and my soon-to-be ex-wife had a sordid history, that you'd come back to town with plans to celebrate a school reunion with, as it turns out, a sick game of mutual bondage and autoerotica that got out of hand.
You seriously misjudged me. Instead of you taking control of my life, YOUR life's about to undergo an amazing transformation. I hope ruining my marriage was worth it, Phillip. I'll be back later tonight to check up on you lovebirds and notify the police of a humiliating discovery.
Turning his head to watch Mike walk away, Phillip tried to scream into the gag but Mike only heard a strangled groan. Mike picked up the bagful of implements, opened the front door, turned and took a bow.