Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh
You are allowed to use this product. Body parts of animals that are deemed halal for us to consume are considered pure; if they are harvested without torturing the animals, they are halal for use.
It is not different from shaving the wool of lambs for use by humans. Allah counts use of wool for humankind as one of His blessings. The case of deer antler is analogous to the use of wool. Enhancing fertility is a legitimate purpose.
In conclusion, you are allowed to use this product if it has been proven to be beneficial or a physician has recommended its use.
I pray to Allah to bless you in your desire to enhance your fertility potential and bless you with a righteous offspring.
Almighty Allah knows best.
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