Jonathan Simpson was shot in the living room of his house in the Brandywell Road area at about 23:00 GMT on Monday.
He sustained gunshot wounds to his thighs and has been transferred to the Royal Victoria Hospital in Belfast. Two men were involved in the incident.
Ch Insp Tony Callaghan said the attack was "callous".  The police have blamed dissident republicans.
Louise Simpson Jonathan's mother said: "I got a phone call to say my son was shot.
"They shot him in the living room and dragged him to the hall and then outside the front door.
"I can't even explain it. I never seen blood like it.
"How can anybody do that to another person? They have no conscience.
"He was in a lot of pain. I thought he was dead."
She added:  "He was taking drugs and I was giving him money for drugs. He was not selling drugs.
"He received threats before. I don't know who is behind them.
"How could anybody put a mother through this."
Ch Insp Tony Callaghan told the BBC: "We think dissident republicans might be behind this attack.
"Even by the brutal standards of previous similar incidents, this was a particularly savage attack.
"Early indications are that this man has sustained particularly severe wounds with potentially life-changing injuries.
"This was not what some people call a kneecapping. This young man has sustained gunshot wounds to his femurs [thigh bones].
"There can be little doubt that these injuries could have killed."
He said the suspected gunmen "made their getaway on foot".
Appealing for information, he said: "In particular we would like to trace the movements of the gunmen before and after the incident."
Deputy First Minister Martin McGuinness tweeted that the attack was "unacceptable".
"There is no place for this in a city looking to the future."
Sinn FÃ©in councillor Kevin Campbell said: "The people of this city do not want these actions taking place on our streets and those behind them need to stop.
"I would urge anyone with any information to bring it forward to the PSNI straight away."
A Londonderry mother whose 23-year-old son was shot several times in both legs, has said she thought he was dead.