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A couple of months ago, I saw a YouTube video, stating that it is still not known how some common drugs really work. I think it might be Healthcare Triage, and I think it might be about antidepressants/SSRIs, but I can't find the video now.

Is is possible for the FDA to approve new drugs, solely on proof that they "work"? ("Work" meaning something like "don't kill anybody, do what they claim to do under known conditions, ...")

If so, what are common examples for such drugs?

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    I'm not sure whether they qualify, but I have read multiple tiimes the claim that the workings of general anasthethics, for example, are not well/completely understand. – Muschkopp Apr 8 '16 at 8:27
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    I'd like to point out that the most they will have is a theory. They could be completely sure and still be wrong. So the question matters less than you might expect. – PointlessSpike Apr 8 '16 at 12:40
  • @Muschkopp that's true. The mechanism by how anaesthetics work is not really understood - all they can really say is that for a person of X weight, use Y amount of Z anaesthetic. It explains why some people, including my sister (twice), tend to wake up when they shouldn't while under anaesthesia - because the method used to calculate the dosage is a rule of thumb. – user1666620 Apr 8 '16 at 13:02
  • Reason I'm asking is because "We don't have to understand everything!" seems to be a common "argument" for homeopathy. Seems that they are right. But you have to show that it works. – stuXnet Apr 9 '16 at 12:00
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According to the FDA the question they want to answer at a new drug approval is: "Does this drug work for the proposed use?"

To answer that question the FDA requires the drug company to submit studies that prove the drug to be safe and effective for the proposed use. There are a host of criteria's such as preregistration that the FDA takes into account but the main criteria for effectiveness is that there are two drug trials that show statistically significant effects for the drug.

Drug design usually starts with a Pharma company deciding that there a target where they believe that a drug that binds to the target will have a positive effect on a condition. Then the Pharma company tries lot of different molecules in High Throughput Screening (HTS).

When it finds molecules that bind to the target they test the drug in animal to see what it does. The test the dosage that's required to kill animals.

If that step is promising they move on to human studies and go through the 4 phases of the the FDA process. The drug company usually has a theory of how their drug works but in most cases the theory will be proven wrong and the drug is not shown to be safe and effective in the clinical trials.

But even if the drug is shown to be safe and effective, that doesn't mean that the theory of the Pharma company is right. Drugs usually do more than one thing. In the case of SSRI the theory based on which they were developed in that they are supposed to affect serotonin levels in the brain.

Further research found out that they also have effects in the gut. The gut also uses the neurotransmitter serotonin. It even uses 95% of the serotonin in the body. Chronic inflammation is linked to depression. In vitro SSRI have been shown to exert significant immunoregulatory effects.

Nobody knows for certain whether the clinical effect of SSRI's on depression shown in the studies that were submitted to the FDA are due to effects fo the drugs in the gut or effects in the brain. It would also be possible that the effects are due to a process that's yet to be uncovered.

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    I don't think this answer satisfactorily answers the question. It dances around the answer but never arrives at it. To be complete, this should discuss why the goal of finding safe effective drugs doesn't need the pathway of effect to be known, and how many drugs today and historically have no known pathway or have disproved pathways of effect. I'd also excise the discussion of side effects, as it is unrelated to the question at hand. I'll be happy to reverse my -1 if corrected. – DampeS8N Apr 8 '16 at 17:03
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    @DampeS8N : (1) I'm not discussing "side effects". There are voices who think that SSRI work because they reduce inflamation (which correlates with depression) by acting on the gut. If that's how SSRI's fight depression that's no side effect but a main effect of SSRI's. (2) A drug having a known pathway doesn't mean that we know how the drug works. A drug interacting with a known pathway doesn't automatically mean that the pathway is responsible for the clinical effect of the drug. – Christian Apr 8 '16 at 18:10
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    None of what you responded with is evident in your answer, they are what I desire to be in the answer. Explaining why the gut would have anything to do with depression seems important to understanding why it is related. – DampeS8N Apr 8 '16 at 18:58
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Yes, it's known that some drugs work, but how they work is unknown.

For example, acetaminophen (a.k.a. paracetemol). MedScape explains:

The exact mechanism by which acetaminophen produces its analgesic and antipyretic effects remains undefined.

Wikipedia explains the general case as "unknown mechanism of action".

These drugs are FDA approved because it's known that they work, their dosage, side effects, and so on. But how they really work is not clear.

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You don't have to look far to find examples of major classes of drug where we don't have a good knowledge of how they work.

One of the earliest "modern" medical advances was anaesthetics, first used in 1842 to transform surgery. Despite using them for a century and a half, medicine is still debating exactly how they work. There is an excellent summary on wikipedia which summarises the situation like this (my emphasis):

It has long been believed that general anaesthetics exert their effects (analgesia, amnesia, immobility) by modulating the activity of membrane proteins in the neuronal membrane. However, the exact location and mechanism of this action are still largely unknown although much research has been done in this area. There are a number of theories, both outdated and modern, that attempt to explain anaesthetic action.

For example, for a long time it was assumed that anaesthetics disrupted lipid bilayers in cell membranes (based on correlations with oil solubility and potency, the Meyer-Overton correlation). But this fails, for example, to explain why stereoisomers have different potency. New theories about specific protein interactions were proposed in the 1980s by Franks and Lieb. But, although this is an interesting theory:

protein targets for anaesthetics have been partly identified the exact nature of general anaesthetic-protein interactions still remains a mystery.

The situation for the mechanism of action of one of modern medicines oldest drugs remains a mystery despite them being in routine use for a century and a half.

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