Stepdad's Serum

By Sir Skinny published March 13, 2019

A step father takes interest in his son’s muscle development.

Hello everyone! Please make sure that you are of legal age before reading this. This story contains adult sexual content involving intercourse between two male adults. For other content information, please see the story tags. If you are 18+ years old (21+ in some areas) please continue, and enjoy.

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My step father has always had a close relationship with me. He has been in my life since elementary school and practically raised me. Two years ago when my mom died, he became my sole guardian.

Last year I began weight training classes at my high school. It was a special senior elective and it allowed me to spend time focusing on myself. My stepdad was really supportive of this. He thought it would build character and allow me to grow into myself. I believe he called it “taking initiative” and said he was proud of me for “becoming a man.” I was surprised that he was so supportive. As a biochemist, he always seemed to place academics above all else. Also, the 48 year-old, 5’9” ~200 lbs man seemed to take no interest in developing his own body. He let his paunch-belly hang over his waist and let his body hair grow out wherever it took root. But I didn’t question his approval, since I love making him proud.

My eighteenth birthday was in early October of my senior year (two months after my weight training class started), and I had a modest party at my place with a few friends. After cake was served, presents were exchanged, and good times were had, everyone went home, and my stepdad and I were left to clean up.

As I was finishing with wiping the table down, my stepdad asked me to come to the garage to see his final gift. Being the teenager I am, I was excited to get some new wheels–maybe a Mercedes or something cool.

Trying to contain my excitement, I followed my stepdad closely to the garage door. He put his rough callused hands over my eyes, opened the door, then removed them. I was shocked to find, not a Mercedes, but a fully stocked, professional gym in place of the disheveled car barn that had existed before. Even though it wasn’t the sweet ride I had been hoping for, I was surprised and delighted to see the workout equipment. My stepdad asked if I liked it. I told him that I love it. But that wasn’t the end of the surprise.

“I have noticed your progress with weight training and muscle development so far, and it makes me proud,” he told me. “So, not only did I get you a home gym, but I have concocted a serum that will make you bypass typical plateaus and push your body to grow and develop muscle as quickly as possible.”

I was shocked! I was also a little scared. “I don’t know…” I said. “That’s very kind of you, but I don’t know if I want to turn to drugs.”

He tried to convince me that it wasn’t dangerous as he summoned the blue liquid-containing syringe from his pocket, explaining that he tested it extensively in his lab. After I expressed further hesitation, he said “that’s fine I guess. I can just return all of this equipment and get you some flashy gift. I just thought that you were taking this thing seriously.” I could hear the sadness and disappointment in his voice.

“Wait!” I called out. “Maybe we can just try it for a while. You went to all this trouble.” I saw a smile appear in the middle of my stepdad’s goateed face.

“Just bend over the bench over there so I can inject this into your fatty tissue,” he told me. I did as I was told, and I felt his hand slowly pull down the waistband of my athletic shorts and boxers. After my right butt cheek was rubbed with an alcohol wipe, I felt the needle pierce my flesh, followed by the simultaneously cool, painful, and tingling feeling of the serum being injected into my ass.

And so began my weekly injections of my stepdad’s serum. He would check my progress once a week before filling me with his concoction. Under his supervision, I grew 30+ lbs of muscle, becoming the biggest, buffest guy in my school.

By the time I graduation came around, I looked like a beast. My stepdad celebrated my graduation by taking me out to a fancy restaurant, taking me to the movies, and finally buying me that car that I wanted. He was especially happy that I would be attending the state flagship university that he worked at in the fall. This meant I would be able to stay with him instead of living in the dorms.

After our celebrations, he took me into our home gym for a body examination and injection. Only this time he had a surprise for me. Instead of the blue vial that I was used to, he brought out a purple one. “We’ve progressed as far as the initial serum would allow,” he told me. “This is the formula that I have developed for stage two.” The rest of the examination went as normal. I stripped down for my stepdad and he took measurements of my body. It wasn’t until the following week or so that I would feel the new formula’s effects.

After working out for a week, I noticed major developments in my pectoral muscles as well as my glutes. It seemed that no matter what exercises I did would grow these areas of my body. My shoulders and arms continued to grow and my waist continued to shrink as well, making my chest and ass appear more prominent.

Despite expressing my concerns at the next examination and injection, my step father seemed delighted at my progress. He assured me that stage two was going along swimmingly. He loved the size of my pecs and glutes in comparison to the rest of my body.

In the following weeks, my pec and glute developments continued along this path. As they began to grow, they became more sensitive. Eventually, I began going around the house shirtless to prevent fabric from irritating my almost constantly erect quarter-sized nipples. This did nothing to prevent the irritation that I felt along my ass crack and hole. After embarrassingly revealing this info to my stepdad, he purchased some jock straps for me to wear around the house. After all, it was summer, and my stepdad and I were the only ones home.

Another side effect of my newfound hyper-sensitivity was irritation as a result of body hair. Even though there was little fabric for it to brush up against, my body hair began to irritate my skin as it sent uncomfortable tingling sensations all over my body with the slightest movement. My stepdad’s solution for this was to shave my body at each measurement and injection session.

When he first coated my giant chest with shaving cream it sent shivers down my spine. Then, when the razor made contact with my flesh I let out a yipe. Almost instinctively, my step father reached up to my nipple and began gently caressing the sensitive areola with his thumb. This transformed the startling sensation of the razor into a soothing one. My squeals turned into moans.

Weeks turned into months, and my newly hairless muscular body began to crave more attention. I asked my stepdad to increase his measurement sessions to once a day and he obliged. I also asked him to include a massage after each, which he also enthusiastically agreed to. During each massage, he knew to dedicate extra time to handling my chest and nipples. They craved attention.

One day, I mustered up the courage to ask him to put his mouth on my nipples. I felt so embarrassed, but he agreed without hesitation. It started with delicate kisses which turned into licks, which eventually turned into full-on sucking. I felt so sorry for subjecting this older, heavier man to nursing on my hairless teenage nipples, but it felt so good. His facial hair tingled my pectoral whole his lips and tongue stimulated my areola. Weirdly enough, I was so turned on by the whole situation. But instead of pitching a tent in my jockstrap, my hole began to pulsate and quiver. I never experienced any sensation in my ass like that before. It was almost as if my anus had become my sexual organ in place of my cock. I began to discreetly finger and play with my hole while my stepdad worked my nipples, but this didn’t go unnoticed for long.

When he paused to take a deep breath, my stepdad noticed my finger playing with my anus. “Hey! What’s going on here?!” he said with shock and almost horror. I began to stammer and say that it wasn’t what it looks like. But my stepdad said, “so you’re a fag then? This is all about sex to you? After all I’ve done for you?” I continued stammering but he went on. “I guess you’re going to go off to college and whore yourself around the dorm now. You’re going to use me and my equipment and then become a fuck toy for some freshman fag, is that it? Shameful.”

I was so embarrassed and ashamed. I kept apologizing and saying how much I love and appreciate him. I said that I would do anything to gain his trust again.

After a few minutes of begging, my step father broke his silence and calmly said, “well, I guess it’s my job as your guardian to provide for you. I really do care about your development. But it seems like you need something more. I guess the only right thing to do is to take care of your needs myself.” I stared at him with confusion. He clarified by saying, “if someone is going to fuck you, it should be me.”

My hole quivered once again at those words. But I was still feeling embarrassed. “I don’t know…” I started.

“Look it’s my job as your stepdad. Besides, it’s the least you could do to repay me for my time and patience.” With that he ran his pale, hairy arm up my tanned, hairless, muscular leg and began to play with my hole.

I began to open up for him and moan as he tickled my opening. He took this as his cue to bend over and continue nursing my pecs. With this, my whole body entered a state of pure ecstacy. I began encouraging my step daddy to continue on. The more vigorously he fingered me and sucked on my nipples, the wetter my hole became. I didn’t even notice him jerking his cock this hole time.

Suddenly, in one swooping motion, my step father released from my teat, pulled out of my hole, and flipped me onto my stomach. I felt the older man’s hands push down on my lower back then pull my hips upwards before jerking his cock twice and entering my ass. Like my weekly serum injections, this caused a combined sensation of pain and tingling pleasure to ripple throughout my body. As my stepdad leaned forward, his large hairy belly filled the small of my back. He layed down on top of me as he completely filled my hole with his thick cock. I felt his warm breath on the back of my neck as he reached around and clasped his hands onto my meaty pecs. “This is my body, boy. I built this body and I’ll claim this body,” he whisper-grunted into my ear. With that he began pumping his cock in and out of my hole. For some reason, this all felt right. I was strangely turned on by my fat, hairy, older stepdad. I needed him to fill me up. He kept pumping into me for the next 15-20 minutes, the whole time whispering filthy things in my ear, and even though I was being degraded and could have easily overpowered him, it was what I needed. He was just fulfilling my needs. Eventually, he came to his climax. He told me that he was going to cum in me. As he began filling me, my stepdad said “that’s a good boy. Take daddy’s seed. Let me breed that hole. I’m just impregnating you will my babies. Good boy.” It was in this moment that I knew I would be my stepdad’s muscular fuck toy for the rest of my life. I finally understood my true purpose.

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