What Causes Severe Hair Loss On The Crown?

Male, age 23 starting to get thinning of hair on crown. weight 70kg, height: 176cm, No medical history of note. I've investigated hairloss products but most seem to have chances of side-effects such as decreased libido which I'm not prepared to take. Also any improvement disappears after you stop using them. So far I can't find anything negative about Procerin, are you aware of any potential side-effects I should be wary of,
Thanks