Learn English – What does ‘chemical hooks’ mean

hook

A curved device used for catching or holding things, especially one attached to a surface for hanging things on. (Cambridge)

addiction

The need or strong desire to do or to have something, or a very strong liking for something. (Cambridge)

The key here, is in the sentence you have quoted. The chemical hooks are the cause of:

...your body physically crav[ing] the drug ferociously.

Addiction (per above definition) is:

The need or strong desire to do or to have something...

Whereas a hook (per above definition) is:

...used for catching or holding things...

Chemical hooks is a reference to the way heroin effects the brain (it works chemically), creating hooks which catch and hold the user to want more of the drug.

The chemical hooks are the means by which someone would be caught. Once that someone is caught (on the drug) then you could say they have an addiction, as they would have a strong desire to do or have (the drug).

How does heroin chemically hook the user? By altering their experience of pain and pleasure as explained by heroin.net:

The effect heroin has on the brain was not well understood until 1972, when a group of researchers from Johns Hopkins University revealed that the human brain possesses specific protein receptor sites for opiates, including codeine, opium, morphine and even heroin. Not long after these opiate-specific receptors were identified, researchers also discovered that the body produces its own endogenous opioids, including dynorphin, enkephalin and endorphins.

When opioids bind to these opiate-specific receptors within the brain, they are able to decrease the body’s perception of pain and elevate mood by increasing levels of dopamine. When the body is introduced to heroin, the body gets flooded with these opioids, leading to enormously high levels of dopamine.

So, in short, the chemical hooks are not synonymous with addiction. The chemical hooks are the means to the end, and the end is addiction.