Please recommend a scalp care ampoule.

My hair has recently become thinner, and my scalp has become itchy and flaky. I'm thinking about trying a scalp care ampoule. Could you recommend which ingredients in commercially available scalp care ampoules are most effective for preventing hair loss and soothing the scalp? I'd also like to know the criteria for selecting a scalp care ampoule suitable for sensitive and oily scalps, and when it's most effective to apply it, before or after shampooing. Finally, I'd also like to know how long it takes to notice changes with consistent use of a scalp care ampoule.