There's no doubt about it, meds can challenge our metabolism and so much more. With so many systems in our body, it becomes a matter of 'Will all my systems adjust well to this med?' Only time will tell.
I can only imagine what it would be like regarding your partner having a child with someone else. Life would also definitely be tough with certain boundaries that come with raising someone else's child. Myself, I'm Mum to 2 amazing creatures (17yo daughter and 14yo son). They are absolute stars who have the challenging task of raising me. By the way, I also raise them.
To be honest, someone could have dropped them on the doorstep years ago and I would still see them as the amazing gifts that they are. Doesn't matter where they came from. I see them as unique individuals, not with heritage or the DNA of their father and myself. They love me for this way I see them.
Every time they raise me through a smile, that smile comes from them not their parents. Every time they raise me through the way they view the world, that viewpoint comes from them trying to work out how the world ticks. Every unique painting ever created was theirs alone. Well...the list goes on. All I have to do is raise them to be their natural self, as they help me return to my natural self. In the process we naturally have fun, navigate the challenges in life without harsh judgement of one another and we naturally and wholeheartedly love each other without conditions.
I try my best to raise everyone around me, whether they're my child or not because I see their individual potential beyond where they have been or where they have come from.
Perhaps your step son is exactly the gift you need in order to remember how truly incredible life can be with all its adventure, unconditional love and beautiful, intelligent and innocent conversation free from the influence of that which comes with age. What you share with him, in you raising each other, has the potential to far outweigh where he came from. You sound like a beautiful sensitive person, a gift to him.