Medications that affect different groups of people in opposite ways- how?

Sorry for the awkward thread title, please allow me to explain.

I’m the weirdo that reads the entire monograph that comes with medication, and if the monograph from the pharmacy isn’t detailed enough, I google it. I want to read about every possible side effect, however uncommon. I’m either a hypochondriac or just really really feel the need to be totally informed.

In doing so, I came across something interesting. Althought weight changes were not a common side effect for this particular medication, they did occur. But it explained that for participants with a low or average BMI, they gained weight. Participants with high BMIs lost weight.

How does this happen? There have to be other medications out there that have a similar effect, I just wanna know why it happens.

(The medication was Geodon, for the curious)

What med is it?

There are others…
Vicodin (opiates in general) make some people drowsey. Both my MIL and I will get all wound up if we take them. (I usually end up cleaning the house all night).

Stimulants will make hyper people calm and calm people hyper. It actually makes sense. In a way it’s doing the same thing to both people it’s just how we (ADHD) react to it versus other people.

This is a very large question and I don’t think we are going to get very far unless it is narrowed down greatly and even then probably not so much. A lot of my undergraduate and graduate school work was in psychopharmacology. It is fairly common for psychiatric drugs to induce the exact opposite of the intended effect in some people. This is called a paradoxical reaction. Anti-depressant drugs like Prozac cause some people to become severely depressed and suicidal for example. There simply isn’t enough knowledge about how the brain works to know why this happens. In fact, most psychiatric drug treatment has to be done through trial and error because variability among people is so high.

Surely this is the answer - that the way in which the body processes medicines varies so widely among people; and that the reasons why are poorly understood.

I think it’s also because the underlying cause of the disease is not understood. I just attended a lecture (I am not a scientist, it was for the general public) where the symptoms of a mental disorder in various subjects were the same, but it turned out that either an excess of some protein or a lack of it cause the same symptoms. So (this is my take on it) two people who seem to have the same disease and are given the same medication can have opposite reactions, the reason being the underlying cause.

I am sure there are other reasons for it as well.

Disclaimer: I am not a doctor, and I don’t play one on TV. I probably shouldn’t be answering this early in the thread.

My father was one of those wacky people who, if there was an unusual or unlikely side effect to be had, he would get it - neuroleptic malignant syndrome, for instance. I tend slightly that way, but not quite as bad - when I started Lamictal, the doctor had to page through the big book of medication info for about a half hour to find my breast pain listed as a low risk possibility. I agree with NinetyWt - the human body is complicated, and some people are just chemically a little bit off the averages.

WAG that some drugs don’t specifically increase or decrease an effect, but rather regulate it, which might mean effects in both directions.

Also, some genetic mutations affect how groups of people’s bodies metabolize or use different substances. E.g., differences in cytochrome P450 affect how alcohol and some other drugs are metabolized, leading to different outcomes.