First, know that there are at least a couple different kinds of powers of attorney, and lots of confusion over the semantics and meaning of all of them.
One is And the named healthcare agent has no legal power to make decisions about other matters such as handling finances or selling property.
a power of attorney for healthcare in which a person can name a trusted individual to supervise medical care or to ensure that specific wishes for that care are carried out as expressed. The person named is generally referred to as a "healthcare proxy" or "healthcare agent." The agent is authorized to step in and speak about medical wishes only if a patient is unable to communicate them for himself or herself.
In a power of attorney for finances, another person, an "attorney in fact," can be named to handle finances. The scope of these documents vary wildly -- from broad powers to handle all financial matters to very limited powers, for example, to pay and file taxes but nothing else. The financial attorney-in-fact has no power to make other decisions for the patient, such as type of medical care.
A minority of all powers of attorney are "durable"-- meaning that they take effect as soon as they are finalized and remain in effect if a person becomes legally incompetent. A short-term memory loss will not usually trigger other types of powers of attorney, particularly if the condition is being successfully treated.
If you're up against the common scenario in which an overly well-meaning or downright meddlesome relative or friend tries to take over too much, check the actual terms of the document and make sure no bounds are being overstepped. If it's a durable power of attorney, and the patient objects or the agent is not acting in the patient's best interest, encourage the patient to change the document to set out limits to act or to name a different attorney in fact.