There are certainly a lot of pills that claim to make your beard or hair thicker, unfortunately they don´t really do anything. Most contain Biotin, which will help restore your growth Level to normal if you are sick or severely depleted of Biotin, once your levels are sated (which they are on almost any diet) it doesn´t have any effect. There are a few products that will make your beard appear a bit fuller such as beard oil, balm or certain shampoos and conditioners. They all coat the hair with a layer of some sort which will make it appear fuller. Then there are things like Minoxidil, which can have serious side effects.

I have written a bit about different beard products here:
moustache cups and mugsbeard brushes and combs
beard literature | simple homemade beard oil

my beard journey | my bart * aber herzlich beard oil | Bartöl store (external link, not affiliated with or endorsed by beard board)