The provision allows an adult Washington resident who has a six month predicted survival to self-administer lethal medication prescribed by a physician.
A recent survey found that only eight terminal patients had taken lethal medications since last November - an indication the law is not being abused.
The patient would also have to take the lethal medication themselves, although a doctor would prescribe it and be on hand during their final moments.
They would then have to wait two weeks before being given the lethal medication in case they changed their mind.
Euthanasia, where a physician or another third party administers a lethal medication, is illegal in every state.
In a report on the first year of its assisted-suicide law, Oregon said that 15 people ended their lives with lethal medications in 1998.
What if they're at the bedside of someone who has taken lethal medication?
The law allows terminally ill patients to receive prescriptions for lethal medications that they must administer themselves, without assistance from a doctor or anyone else.
"The least costly treatment for any illness is lethal medication," he added.
At times, it may be important to limit access to potentially lethal medications for patients considered at risk for suicide.