I was having a discussion with friend and he said a lot of medication have side effects. I’m confused of why lot of medication have side effects? Is this because medication is not target therapy?

Will quantum computers, AI and super computers allow for more targeted therapy?

Why do treatments cause side effects in the body?

The body is complex. Think of it as like the global weather system. Organ systems interacting, complex metabolic pathways, complex biochemistry, our cells interacting with bacterial symbionts in the gut, immune system interacting with invaders and with our own tissues, etc. Tinkering with all that is somewhat like tinkering with weather - all kinds of unforseen consequences emerge. It would be weird if there weren't side effects. Even a drug like acetaminophen (paracetamol, Br.) which seems gentle and harmless can adversely affect liver function in some people. Ibuprofen can be hard on the kidneys. Dosing many drugs is based on the good outweighing the harm.

Edited January 9Jan 9 by TheVat

I don't think of it as the medications having side effects. The meds are designed to do what they do for the majority of patients. It's the patients who have differing issues and physiologies. Their bodies function slightly differently from one another, or react in varying ways, so the meds still do what they're supposed to, but some folks experience "side effects". Some of these meds list a dozen or more possible side effects, and I'm betting almost nobody gets them all.

Perhaps in the future we'll have diagnostic techniques that can prescribe a more precise, consequence-free form of medication. Socialized medicine would be perfect for this, since for-profit healthcare capitalizes on side-effects and cheap pharmaceuticals.

It’s not reasonable to expect otherwise.

Chemicals/drugs have effects. If one effect is that it fights a disease, we are tempted to use it. But why would it have only that effect? What we hope for is that the effects other than fighting a disease are tolerable. If not, the drug is not used.

2 hours ago, Moon99 said:

Will quantum computers, AI and super computers allow for more targeted therapy?

Quantum computers aren’t really geared toward that, should they ever scale up. Supercomputers are undoubtedly already being used. AI, too, but it would be machine learning to analyze data. LLMs can only sift through what they’ve been trained on, so probably not. Neither they nor quantum computers are magic.

Why do medication have side effects?

Everything consumed in excess has undesirable side effects. Drinking more than 6 liters of water a day can lead to death (for people weighing less than 75 kg).

Edited January 9Jan 9 by Sensei

57 minutes ago, Sensei said:

Everything consumed in excess has undesirable side effects. Drinking more than 6 liters of water a day can lead to death (for people weighing less than 75 kg).

In excess? I think when the OP said "a lot of medication", they meant "many medications on the market", not an excess of one. Or are you claiming the dosages are excessive in general?

But what do these drugs do like how do drugs work that it has side effects.

30 minutes ago, Moon99 said:

But what do these drugs do like how do drugs work that it has side effects.

Answers on the Internet: Explainer: how do drugs work?

That's a great intro, with clear helpful illustrations to explain receptors and agonists. I haven't delved into the cannabinoid receptors much, but that seems like an interesting topic, especially as to what are the natural agonists for that receptor. They are called endocannabinoids.

12 hours ago, Genady said:

Answers on the Internet: Explainer: how do drugs work?

It talks about receptors and agonists and how drugs work but it does not explain why drugs have side effects.

52 minutes ago, Moon99 said:

It talks about receptors and agonists and how drugs work but it does not explain why drugs have side effects.

Here is an example: aspirin: https://en.wikipedia.org/wiki/Mechanism_of_action_of_aspirin

If you read that, you will see that it inhibits the production of things called prostaglandins, which have more than one role in the body, two of them being a role in the body’s inflammatory response and a role in blood clotting. So if you take aspirin for inflammation, you will also bleed more if a blood vessel is broken. That’s a side effect, especially gastro-intestinal bleeding , which can be an issue if you take a lot of aspirin. However that also means aspirin can be used to prevent unwanted blood clotting, e.g. in heart attacks.

This is typical of the way many drugs work. They alter something in the biochemistry of the body, which gives one desired effect, but often they have other consequences as well, which may or may not be a problem depending on what is and also on the condition of the patient.

Good explanations all around. Perhaps the OP can look up something like cancer chemotherapy which is a fairly dramatic example of side effects which tend to affect most patients and in a miserable way. Plus side: maybe you get to live some more years. And it's another example of therapy where you want to do something mean to the cancer cells but it's going to be in the bloodstream and also affect normal tissues.

5 hours ago, exchemist said:

Here is an example: aspirin: https://en.wikipedia.org/wiki/Mechanism_of_action_of_aspirin

If you read that, you will see that it inhibits the production of things called prostaglandins, which have more than one role in the body, two of them being a role in the body’s inflammatory response and a role in blood clotting. So if you take aspirin for inflammation, you will also bleed more if a blood vessel is broken. That’s a side effect, especially gastro-intestinal bleeding , which can be an issue if you take a lot of aspirin. However that also means aspirin can be used to prevent unwanted blood clotting, e.g. in heart attacks.

This is typical of the way many drugs work. They alter something in the biochemistry of the body, which gives one desired effect, but often they have other consequences as well, which may or may not be a problem depending on what is and also on the condition of the patient.

So if I understand it medication changes molecule or protein in your body? And these molecule or protein do more than one thing? So they want to make drugs that change the protein or molecule but it does more the one thing the protein or molecule?

48 minutes ago, Moon99 said:

So if I understand it medication changes molecule or protein in your body? And these molecule or protein do more than one thing? So they want to make drugs that change the protein or molecule but it does more the one thing the protein or molecule?

Yes. Though often I think the effect of a drug is found before the mechanism is established. So it is not a clean process of looking for the molecule to alter and then making a drug to do so. Certainly with older drugs like aspirin, the effect was known long before the biochemistry by which it works was understood. But the essential point is that biochemical molecules in the body often do several different things, so affecting them can have more than one consequence.

Or else, as with @TheVat 's example of chemotherapy for cancer, the aim is to disrupt the process of cell division which is very fast and out of control in cancer. So it disproportionately disrupts the growth of the cancer cells, but with the side effect that it also disrupts the cells that are dividing normally and generally makes the patient feel ill. Basically the patient is being poisoned, but with a poison that hurts the cancer cells worse than everything else. So you give a course of treatment, during which patient feels iller and iller from the side effects - and then have a pause to allow the normal cells to recover from the attack, before repeating the process. It can be pretty exhausting for the patient but, you hope, it kills off the tumour(s).

Edited January 10Jan 10 by exchemist

In general terms, the action of drugs/poisons is extremely complex. There really is no simple answer to the question of why drugs have side effects. An inkling to the complexity of drugs/poisons can be gathered by examining the Wikipedia article on the 5-HT receptor. This is a receptor that is activated by the neurotransmitter serotonin (5-hydroxytryptamine). The thing to note is that there are 14 known distinct 5-HT receptors in 7 families, and that these receptors are distributed throughout the body in locations as diverse as blood vessels, central nervous system, gastrointestinal tract, platelets, peripheral nervous system, and smooth muscle. Drugs/poisons that bind to 5-HT receptors can do so as agonists (full or partial) or as antagonists, and can bind to the different 5-HT receptors with different affinities, perhaps even acting as agonists on some receptors but antagonists on others. Thus, different drugs/poisons that that bind to 5-HT receptors can have quite different effects, and even different drugs/poisons that bind predominantly to a particular 5-HT receptor as an agonist may exhibit differences in effects.

Edited January 10Jan 10 by KJW

18 minutes ago, KJW said:

In general terms, the action of drugs/poisons is extremely complex

I think it’s a good point to include poison in the discussion, because drugs are potential poisons, we just use them in nonlethal doses. Botox would be an example that clearly shows this. It’s the botulinum toxin, perhaps the deadliest natural substance we’ve discovered (nanograms per kg of target is a lethal dose if injected) but sufficiently diluted it’s used as a treatment for certain afflictions.

It’s a matter of what it does, and whether that helps with some medical issue if you use the right dose and delivery method (injection vs inhalation vs oral intake generally have different lethal doses)

21 hours ago, TheVat said:

I haven't delved into the cannabinoid receptors much, but that seems like an interesting topic, especially as to what are the natural agonists for that receptor. They are called endocannabinoids.

One thing I learnt fairly recently is that Δ⁹-THC found in cannabis is a partial agonist and therefore is self-limiting in its effects, whereas synthetic cannabinoids such as JWH-018 are full agonists and therefore are potentially more dangerous.

23 hours ago, TheVat said:

That's a great intro, with clear helpful illustrations to explain receptors and agonists. I haven't delved into the cannabinoid receptors much, but that seems like an interesting topic, especially as to what are the natural agonists for that receptor. They are called endocannabinoids.

Because it isn't exactly like a key in a keyhole, it is messier than that, and more things fit the keyhole than one exact chemical shape from what I understand . The cell isn't matching the whole chemical, but even just a side of the chemical.

The keyhole being receptors and key being agonists.

15 hours ago, Moon99 said:

Because it isn't exactly like a key in a keyhole, it is messier than that, and more things fit the keyhole than one exact chemical shape from what I understand . The cell isn't matching the whole chemical, but even just a side of the chemical.

The keyhole being receptors and key being agonists.

Yes, we use the word "analogy" with the understanding that it is an imperfect and simplified model of what occurs at a cell membrane. Hopefully, the discussion has underscored that. Relevant to this also is what @KJW mentioned, that there are partial and full agonists, and their effects can differ.

As mentioned, side effect is a bit of a misnomer, drugs have effects, some beneficial, some less so. A more appropriate terms would be "desired effect" for a particular treatment. Sometimes, a side effect for one treatment, can be a desired effect for another condition.

22 hours ago, Moon99 said:

So if I understand it medication changes molecule or protein in your body?

Not quite, what most medication does is to interact with molecules in your body (including proteins) and change activities. They can inhibit or activate functions and thereby alter biochemical processes in some way. For example, let's say your body doesn't produce enough thyroid hormones, you can supplement your body with medication. However, normally your body modulates the synthesis rate depending on internal signaling so that it remains somewhat in homeostasis (depending on your activity diet and other factors). If you take the drug, the dosage is fixed, i.e. after you take it, the levels are high and then go down over time until you take a new dose. Depending on how well the dosage is dialed in, you may have higher or lower metabolic activities than you would like as a consequence. The results of that (e.g. increased or decreased appetite, weight increase or decrease) could be considered side effects, but are in fact the primary effects of the drug.

On 1/11/2026 at 5:27 PM, CharonY said:

As mentioned, side effect is a bit of a misnomer, drugs have effects, some beneficial, some less so. A more appropriate terms would be "desired effect" for a particular treatment. Sometimes, a side effect for one treatment, can be a desired effect for another condition.

Not quite, what most medication does is to interact with molecules in your body (including proteins) and change activities. They can inhibit or activate functions and thereby alter biochemical processes in some way. For example, let's say your body doesn't produce enough thyroid hormones, you can supplement your body with medication. However, normally your body modulates the synthesis rate depending on internal signaling so that it remains somewhat in homeostasis (depending on your activity diet and other factors). If you take the drug, the dosage is fixed, i.e. after you take it, the levels are high and then go down over time until you take a new dose. Depending on how well the dosage is dialed in, you may have higher or lower metabolic activities than you would like as a consequence. The results of that (e.g. increased or decreased appetite, weight increase or decrease) could be considered side effects, but are in fact the primary effects of the drug.

My understanding is drugs don’t necessarily change the protein’s structure. Many drugs simply bind to the protein active site and block the native substrate from binding.

So from what I understand drugs do not change the protein structure And from what I understood receptors are protein.

So different receptors are different protein.

The drug do not binds to the receptor molecules and changes it?

The extent of change could be conformational, but it is not necessarily a full on alteration of their regular structure or function. So "change" is mostly on the activity level.

Those "changes" are also generally transient (with exceptions).