I have had tinnitus for about two years due to singing very loudly along with amplified guitars. I have read a lot about it since.
Firstly get an examination by a doctor to make sure it is not due to infection in your ear. If this is the problem the longer it is left the bigger it will become.
Some drugs can certainly cause tinnitus. Quinnine and asprin are both well known. The mechanism by which they affect hearing is unknown.
Hearing is a very complex perception that extends well beyond the ear. Sound perception relies on an enormous amount of processing to the signal received from the ear. Our ability to hold a conversation against a noisy background it testimony to its extraordinary performance.
There is a fantastic difference between the quietest sound we hear and the loudest. Much of the gain adjustment is done by the brain. Anything that upsets the brain can lead to tinnitus.
In complete quiet many people hear tinnitus. It happens because the brain increases the sensitivity until it hears something. Like electronic amplifiers the higher the gain the more likely the system will go into feedback and we hear a ringing sound.
It is frequently triggered by ear damage but due to the sensitivity of the system even minor damage can upset the balance.
However once it becomes noticed it can lead to the attention becoming excessively focussed. The more you worry the more likely it will set in.
The best information I have seen on tinnitus is at:
http://www.tinnitus.org/
It is really worth reading for anyone with tinnitus.