system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What would happen if someone were to consume an addictive substance like methamphetamine (enough to get addicted to it) without realising they had consumed it?
gpt
There's an important distinction here between psychological and physiological dependence. Some substances are both, some are not.

It's also important to note that *anything* can become a psychological addiction, like sex, gambling, 
If a person were to unknowingly consume a substance that causes psychological but not physiological dependence, nothing would happen. Psychological dependence occurs through repeated exposure to the substance/behavior that becomes interpreted as a reward. If there's no reward that can be identified, there's no psychological dependence. 

Physiological dependence is different. Chemical changes in the body would make that person dependent on the substance in question, for example, nicotine or opiates, whether they knew they were consuming it or not. If they stopped consuming it, they would experience withdrawal symptoms.