Antiviral drugs destroy viral pathogens, but it is difficult to kill a virus without damaging body tissues and this makes them very expensive to develop.
Painkillers just stop an individual from feeling pain. They don't destroy any pathogens, they just make us feel a bit better by removing some of the symptoms.
What about other medicines (ibuprofen and cough mixtures) ?
Ibuprofen reduces inflammation, and cough mixtures just soothe, often by reducing friction in our throat or chest. They remove symptoms but don't destroy pathogens.
Why do drugs need to be trialled before public use?
To check they are safe and effective. They are tested for toxicity (is it poisonous), efficacy (does it really work) and dose (what's the lowest dose that works).
Testing on people, both healthy volunteers, and patients who might benefit from the drug. The first clinical trials give tiny doses to make sure it is safe.
The people in the trial are split into two groups. One group will be given a placebo. The patients in the trial don't know which they are given and nor do the doctors.