Pretty much as in the title. Are there drugs in existence (or drugs that are strongly hypothesized to exist) that do the exact or pretty close to the opposite of existing drugs? For example, one group of Antidepressants are known as “Selective Serotonin Reuptake Inhibitors”. These drugs supposedly lift mood. Are there drugs that specifically lower mood and/or mutually nullify the effect of antidepressants? E.g. perhaps a “Selective Serotonin Reuptake Accelerator”.
Don’t just say “depressants”, because those are central nervous system depressants, not mood depressants.
Other drugs are also in scope - for example, are there “propsychotics”, “proalgesics”, “protussives”, “mood destabilizers”, “beta battering rams”, or “prohistamines”?
organisms are complex biochemical systems with many types of feedback and control. pathways and processes can be be stimulated or diminished from their ordinary level, the system has chemicals for it.
systems will get affected without directly using a drug as well. you can remove a drug affecting a system and get a rebound taking that thing beyond a normal level in the other direction. some people might take an antihistamine for a cold and after the cold is gone and they stop taking the antihistamine get a rebound of histamine levels that is as high as the cold would have done (though maybe shorter).
It’s very hard to design a drug - which is something which we’d have to master in order to be able to answer “yes” to your question.
Drugs come about mostly through observation of how animals (including humans) react to stuff. Historically, this “stuff” is largely plants and fungi and molds - we eventually identified the molecule that created a particular effect and called that a drug to treat X.
Further advances in drugs tend to be either modifications to something like X, maybe adding or removing or slightly changing a functional group (a reactive chemical component of the structure) and seeing if that treats a disease/symptom better or worse than the existing drug. Perhaps the drug metabolism is such that it breaks the drug into two parts, and one part is prone to causing bad side effects… using incredibly simplistic examples, if you can take something that looks like a d and modify it so that it looks like an a, that could be an improved drug.
Using a random and not entirely thorough example off the top of my head:
Designing a drug to treat a particular disease involves knowing how a disease works…if you’re lucky, you have something like the CML leukemias caused by the Philadelphia chromosome, you can identify a single protein/enzyme that needs to be deactivated, and you can find a drug that will do that. That doesn’t mean you can necessarily create something that will “just as easily” enhance/activate the protein (if you consider the original development to be “easy”).
If you’re unlucky -as is the case with most diseases you could name, and several more you couldn’t - there isn’t a single protein/enzyme or disease source that you can target in order to treat the disease, let alone conceive of a designed opposite.
This doesn’t address the idea of why would you want to? Despite lots of web pages on the internet, most Big Pharma - and Small Pharma - researchers really do want to just treat diseases. It just isn’t all that simple.
And as johnpost said, you do have “opposite” effects and side effects. You can have “uppers” and you can have “downers” or you could have drugs that cause constipation and drugs that cause loose stools, and you could label those as opposites as you suggest in your OP, but there isn’t much point in doing so, other than perhaps recognizing that these effects can be used to balance a drug dosing regimen in order to minimize the side effects/suffering by the patient!
Lets say a drug causes an effect because it has a molecule correctly shaped to lock into a receptor in the brain and activate it(these are called agonists) well there are also drugs which have molecules shaped slightly differently so that they lock into the receptor but do not activate it, and they also sometimes block any agonist from getting into the receptor too(these are called antagonists).
Most drugs with a psychoactive effect that are well known like opiates and methamphetamine work like this, see this article: