Yes, and no, so both. After the first use, the sight, and the strange magical energy our characters give off will remain, but the powers of our characters will fade over time. I also think it would be cool if different dosages had different effects. 
For example, one Idea is that the sight will dim a little the longer a person goes without the drug, and the energy he gives off like a beacon will also diminish, but the longer they go without taking the drug, the weaker they'll get, physically and magically. On the other hand, the more of the drug that's ingested the more power they'll be able to summon, at the cost of glowing like a beacon to creatures attracted to the strange energy they produce; plus their third eye will open more and they'll be exposed to a greater chance at being driven insane, and as is the case with all drugs taken, the greater the amount, the harder it will be for you to focus, which could have dangerous effects when gathering magical energy is involved.