Why stairs stand out first
Climbing stairs puts considerably higher forces through the knee than walking on the level. Going down is usually more demanding than going up, because the muscles work as a brake and absorb body weight.
For many people the staircase is therefore the first place where something becomes noticeable that stays hidden on flat ground. That is an observation about load, not a diagnosis.
What direction and timing suggest
Symptoms mainly going down, often at the front of the knee, form a different pattern from symptoms going up. Symptoms after long sitting that ease after a few steps are different again.
These distinctions help in a consultation and save time. They do not replace an examination, because the same pattern can have different causes.
When not to wait
With severe pain after a fall or twist, marked swelling, a knee that no longer fully straightens or bends, redness with heat, fever, or a sense of instability, the knee needs medical examination.
The same applies to symptoms persisting over several weeks or increasing. A capsule is not the right first step in any of those situations.
Managing load day to day
With ongoing irritation it usually helps to redistribute load rather than avoid it entirely: use the lift for several floors, use the handrail, descend slowly and in control, and place the whole foot on each step.
Over the medium term, strengthening the thigh muscles and hip stabilisers is what counts. Which exercises suit you depends on the cause and should be guided professionally.