A man with drug addiction problems was found badly decomposed in his Birdham home, an inquest heard.
Simon Dawson's body was discovered by police at his home in Old Common Close in Birdham on February 7 after a neighbour called expressing concern for his welfare because he had not been seen for a while.
An inquest held at Crawley Coroner's Court on Tuesday heard that the 48-year-old's body was found on a sofa bed in his bedroom with a bottle of cider in his hand and a needle and syringe next to him.
A post mortem held on February 12 could not determine a cause of death.
In her concluding statement, senior coroner for West Sussex Penelope Schofield said: "Mr Dawson was found in an advanced state of decomposition, therefore it is clear he had died some time previously.
"We know he had a clear history of drug and alcohol abuse and it is highly likely that this was linked to his death; however as it has not been possible to provide a formal cause of death, I'm afraid that in this case I will have to leave an open conclusion."
The inquest heard how Mr Dawson had a normal upbringing with five siblings, but got into trouble and began stealing from his family.
He ran away to London and later to Sussex, and had no contact with his mother for 13 years before reconciling after the death of his sister Emma from methadone and alcohol abuse three years ago.
The inquest heard how he would spend around £800 a month on heroin and crack cocaine and £60 a week on cannabis.
He was receiving help from the Sussex Drug & Wellbeing Service in the months leading up to his death, but did not engage regularly with them. The service visited his home once in the lead-up to his body being found, but there was no response.
Mr Dawson was last seen by his mother on January 10 after a visit to the Midlands to see her.
He told his mother 'he could not wait to be with his sister Emma', but when questioned about this, he did not say what he meant because he did not want to worry her, the inquest heard.