Dear Work,
Unfortunately I am unable to attend on Tuesday. I have been diagnosed with a mild addictive disorder which results in depression if the needs of my addition are not met. Symptoms would include lack of concentration, slowed thought processes and make me prone to mood swings. In all, this would seriously impair my ability to do my job properly and could possibly be of detrimental effect to my work colleagues.
To avoid this, my doctor has advised me that I need to complete an addition therapy session on Tuesday. Having this time would avoid any further episodes, increase my overall productivity and has a high chance of success of curing the disorder.
Yours sincerely,
A lying gamer. ;-)