Well, it makes his dick shrink a lot.

As soon as he got the serum he felt a warm sensation in his crotch and his head started spinning, remembering his first transformation he quickly looked for a place to sit down and ended on the living room sofa.

His gargantuan cock was resting on his right thigh, almost hiding it completely. K-T was reassured after a minute or two, the serum was working, his cock was visibly shrinking. Still it was a lot slower process than the growth, but his sister had warned him about it.

After a good 20 minutes his cock was back to his normal 9 inches length and his balls felt a normal size again. He decided to get up and go get dressed, not wanting to be naked when Lily would arrive. There was a problem though, the fatigue and dizziness that accompanied the serum action had not disappeared and he had trouble getting up.

After deciding to stay put for a few more minutes to recover he realized what was happening, the serum was still active! Already he was a good inch shorter than usual, which left him with still a respectable 8 inches cock, but he had lost in girth too. The real issue was that as the time passed the serum wasn't showing any signs of stopping.

K-T still managed to go back to his room to try to contact his sister on his computer, but she had made good on her word of wanting to be left alone and had blocked him on the messaging app they used to communicate.

K-T had dressed in the t-shirt, swim trunks and flip-flops he planned to wear on the beach and was sitting on his bed with the trunks around his ankles. He was anxiously looking at a measly 5 inches cock and at the clock showing him that Lily would arrive in 20 minutes.