The Intoxicated Informer

Charge: sale of a proscribed substance, to wit, heroin, to I. The state introduces evidence to show that I, a known heroin addict, was given $300 by the police to purchase heroin from D at D's barber shop. The state calls I as its first witness. D objects to any testimony from I on the grounds that I is incompetent to be a witness by virtue of drug use. The trial court conducts an in-chambers hearing on the question. D calls P, a psychiatrist, who testifies that the use of LSD may confuse one's perception, thereby impairing the capacity to perceive or remember one's observations. P's opinion is based on her experience with LSD users, whom she has found to have a history of suffering blackouts, although she has not personally interviewed I. I admits to excessive use of drugs, including LSD, but denies ever passing out, freaking out, or having loss of memory from the use of LSD.

What ruling and why? Is there a minimum threshold of the ability of the witness to observe, remember, relate, and appreciate the obligation of telling the truth that the witness must cross before he will be allowed to testify?