There is no benefit to taking this sublingually that I know of. Generally the reason you take anything that way is to get it into your system as quickly as possible or because saliva is beneficial to absorption. With 5-HTP, it takes some time for your system to react to it assuming your body finds it necessary to have more coming in -- if it doesn't, it just evacuates it. The same thing is true when you take medication targeted at serotonin, though they work quit differently. It just takes the brain some weeks usually for the brain to adjust to the new serotonin regimen. Amino acids are usually best taken apart from food because the digestive process can make them harder to absorb. Remember, this drug needs to get past the blood/brain barrier for it to work. I'd just take it as directed. And remember, sublingual substances are either in liquid form or something that dissolves under the tongue. This capsule isn't designed that way.
I use sublingual B12 spray daily. It does take around 5 minutes for B12 to reach the brain. Something I found out dramatically after 3 weeks without B12 (have autoimmune pernicious anaemia). I would assume you need the proper sublingual form of a product for proper absorption.
What's the dosage and brand?