I am only half way through but the chain in the cover picture is metaphorical.
It's about an investigator working on a drug case and younger guy who is presumed to be a junkie and part of the drug distribution but his tests are clean. They end up living together and they have a lot of sex.
The later chapters should explain how is the younger guy related to the drugs.
There is no chains, or collars. Their sex is consensual. There no kinks or hyper dominance/submissiveness, no forced confinement. There is drug taking and aftereffects though. The investigator is the bottom. They have an age gap but both are adults.

I have a bit of ‘trauma’ with chains and collars so can someone give me a summary of this series