Something is driving them to use a drug, and until that underlying force is dealt with, they are not going to stop using. It can be extremely difficult to remove whatever problem caused a person to use drugs in the first place. Most likely, removing the drug producing obstacle can prove to be almost impossible. If it's a person's job, there's not much they can do, they may absolutely need that job to survive and feed their family. So, they will continually be driven to the drugs to alleviate their stress. And once they start using a drug, they will very likely not be physically able to stop, it may be nearly impossible. Especially if they are using a harder substance like cocaine. The brain will begin to physically change, making it impossible for a person to resist their drug of choice. The time to addiction varies from person to person, being dependent on genes, mental and physical health, and environment. A person can become addicted to a drug in a few weeks. Once a person is at or near addiction, your life, and theirs, will quickly lose touch with all reality.
Drugs cost money, and an addict will do anything to get their fix. If a savings account is available, they will start siphoning money off of it immediately. There will be fights about money being wasted, and your partner will just use that as an excuse to use more drugs. The whole scenario quickly spins out of control. If there is not enough money to support the addiction, your partner will do whatever is required to get that money. This often involves stealing or pawning possessions. If they steal, arrest is a very real possibility, and if they start pawning household items, and it works, then your house will soon have nothing in it. Using drugs is often a social activity, and if you're not participating, your significant other will find someone else who will. You will feel like a stranger when this happens; addiction will force your partner farther and farther away from you. This course of events will unfold quickly and painfully. Get as much support from family members and friends as you can, but also get professional support as quickly as possible.