> if there's also a written contract, they overrule any unwritten agreements That typically only applies in situations where the agreement is otherwise ambiguo

> if there's also a written contract, they overrule any unwritten agreements That typically only applies in situations where the agreement is otherwise ambiguous. The most common example being "A: We verbally agreed to $1 and I signed. B: But what you signed say $1.10" often in these situations the written contract win. However in this case they are in a written format explicitly clarifying the contents of the contract which you are allowed to do as long as the interpretation you agree to doesn't meaningfully diverge from what was written in a way that doesn't follow standard practice. As the doctor said it is common practice to avoid consuming alcohol with medicine that has known interactions with it. Even if the original contract is unambiguous no one is going to side against OP on the topic of "we both agreed that it meant when taking certain medicines" since the original verbiage didn't follow standard practice by the clarified verbiage did.