Final answer:
A 91-year-old patient needs to be free from consuming alcohol for approximately two weeks prior to safely starting disulfiram.
Step-by-step explanation:
Prior to safely starting disulfiram, a 91-year-old patient should be free from consuming alcohol for approximately two weeks. Disulfiram is a medication used to deter alcohol consumption by causing unpleasant side effects when alcohol is consumed. The duration of alcohol avoidance is necessary to allow the body to fully metabolize any alcohol present in the system and to minimize the risk of adverse reactions.