Why do drugs that do the same thing have different side effects in the same person?
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Doing the same thing isn’t the same as doing different things and getting a similar result. For example, if you want to have a weight loss medication: you could have one that surpasses your appetite, drug A. This would cause a person to eat less potentially by increasing the production of the hormone that tells you you are full, which results in less incoming calories, therefore a calorie deficit, therefore weight loss. On the other hand, you could have a medicine, drug B, that increases your heart rate and dopamine. Which might cause someone to have a lot of energy and be hyper. Increase in passive muscle activity causes an increase in calorie use, which leads to a calorie deficit, and also leads to weight loss. Now, drug A messes with the hormones that control your digestive tract, so it might also mess with other things like nutrient absorption or stomach muscles (cramping, constipation, etc.) Where as drug B increases muscle activity so you might get heart palpitations, or muscle spasms. Same end result, different routes to get there, different side effects.
Different drugs can achieve the same goal in different ways.
If someone attacks you then you can use e.g. a gun or a knife as self-defense weapons, the goal is the same but the side effects will be different.
I asked my pharmacist this after I had some weirdly bad drug reactions. I’ll try and relay it accurately. I don’t think this applies to all medicines, but at least sometimes it’s something like this:
Your body takes ingested medicine through your digestive system, and it gets filtered through your liver.
Your liver has different types of tools it uses to break down things for your body to get rid of easier, and the amount of each tool is different in each person
Let’s say that you take medicine, it passes through your liver, and it needs both Tool A and Tool B to be processed properly. If you have a lot of Tool A, you can do the first round of processing really fast, and can break down part of the medicine right away. If you have too much, you can do this too quick, and Tool B can’t keep up.
If you don’t have enough of Tool A, the medicine might take a long time to wear down, or it’ll just pass through unchanged. Tool B might not be able to break down unchanged medicine, so it can’t help. You might have the useful bits of medicine get sent away too fast to use
Even if you have the right amount of Tool A, if you don’t have enough of Tool B, things can still go wrong. Tool A could send Tool B too much and overwhelm it, and it could take a long time for the medicine to get broken down and processed all the way. Partly broken medicine could stay working much longer than it’s meant to, and act stronger than intended
If one of the partly processed things causes a side effect, then that one can stay as long as it takes to get rid of the medicine, even if the useful bits have already been used up.
Pharmacologist here—drugs that have the same “action” may be achieving that result in a different way. Essentially drugs are like keys that interact with locks in your body called receptors. Throughout our body we have many different locks and many copies of each type of lock. When the key binds to the lock, it either makes the lock do what it normally does when interacting with the “natural” key, or it makes it more difficult for the lock to work.
Two sleep drugs:
Diphenhydramine (Benadryl) works by binding a histamine receptor in the brain and preventing the release of more histamine, which acts to promote wakefulness. Effect = drowsiness. There are also histamine receptors in the body and when Benadryl binds these, they promote anti-inflammatory mechanisms, to help with decreasing allergy symptoms
Midazolam (a benzo) works by binding a receptor called a GABA receptor in the brain and when it binds, it causes that receptor to work more—the receptor’s normal job is to decrease activity in neurons. For this reason, benzos can be used to promote sleep, to help with anesthesia and anxiety, but one of their scary side effects is respiratory depression, which happens when neurons supplying the diaphragm (breathing muscle) have their activity decreased too far.
Different locks/receptors have different jobs in the body.
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