das Gegenmittel
antidote
A substance that cancels out or neutralizes the effect of a poison.
Nach dem Schlangenbiss wurde ihm sofort ein Gegenmittel verabreicht.
After the snakebite, he was immediately given an antidote.

antidote / remedy
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antidote
A substance that cancels out or neutralizes the effect of a poison.
Nach dem Schlangenbiss wurde ihm sofort ein Gegenmittel verabreicht.
After the snakebite, he was immediately given an antidote.

The word is made up of 'gegen' (against) and 'Mittel' (remedy/means). So it is literally a remedy that works 'gegen' (against) something, whether a poison or a problem.
Gibt es ein wirksames Mittel gegen diese Krankheit?
Is there an effective remedy for this disease?
'Gegenmittel' (antidote/remedy) is almost always used with the preposition 'gegen' (against) + accusative to name what it works against. This pattern applies to both the literal and the figurative meaning.
Die Ärzte suchten ein Gegenmittel gegen das seltene Gift.
The doctors searched for an antidote to the rare poison.
You can tell from the context whether the literal or the figurative meaning is intended. If it is about 'Gift' (poison), 'Schlangenbiss' (snakebite), or 'Vergiftung' (poisoning), it is literal. If it is about abstract problems like 'Langeweile' (boredom), 'Hass' (hatred), or 'Armut' (poverty), it is meant figuratively.
Das Gegenmittel wirkte schnell gegen das Schlangengift.
The antidote worked quickly against the snake venom.
In a medical context, an antidote is often 'verabreicht' (administered), 'gegeben' (given), 'gespritzt' (injected), or 'entwickelt' (developed).
Dem Patienten wurde sofort das richtige Gegenmittel verabreicht.
The patient was immediately given the right antidote.
In the figurative meaning, 'Gegenmittel' (remedy) is often combined with abstract concepts. Typical phrases are 'ein Gegenmittel gegen Einsamkeit' (a remedy against loneliness), 'gegen Ungerechtigkeit' (against injustice), or 'gegen Langeweile' (against boredom).
Regelmäßiger Sport kann ein wirksames Gegenmittel gegen Müdigkeit sein.
Regular exercise can be an effective remedy against fatigue.
solution
'die Lösung' and 'das Gegenmittel' can both refer to something that helps with a problem. 'die Lösung' is neutral and general: it can be a plan, a method, or a way forward. 'das Gegenmittel' sounds more strongly like something that directly counteracts a negative development. It is also used figuratively, for example against stress or boredom. Learners may confuse the words because both express help. For normal problems, you usually say 'Lösung'. 'Gegenmittel' fits when something is meant to specifically stop a harmful effect.

Regelmäßige Pausen sind für sie ein gutes Gegenmittel gegen Stress.
Regular breaks are a good remedy for stress for her.
Für den Streit im Team sucht Herr Klein eine faire Lösung.
Mr. Klein is looking for a fair solution to the dispute within the team.