A sign is something other people, like a doctor, notice while a symptom is something that the patient describes. For instance, drowsiness might be a symptom while dilated pupils might be a sign.
Drug addiction - when an individual is dependent on a substance, like a drug, nicotine or alcohol, he/she is unable to manage his/her use of that substance. Even if the substance poses a danger, they will still take it whether or not they know the dangers.
Being dependant on a substance can lead to strong cravings. It's possible that the addict wants to stop taking the substance but finds it really hard to do so on his or her own.
The condition of the person, their family lineage, the substance in question, and the person involved are some of the factors that determine the possible signs and symptoms of addiction.
The clues and indicators of addiction could be that:
A person, because they indulge in alcohol or drugs may exhibit some or all of the above symptoms and not be certified addicts, but they will not display any of the withdrawal symptoms or the associated craving.