Chelsea Cameron's parents were in the grip of their addiction when they missed her younger brother's first day at school and the day she was made head girl.

But in a heartfelt letter she has posted on her blog, the 18-year-old from Dundee does not criticise her mother and father for their absence.

Instead, she thanks them for teaching her to be independent, ambitious and to steer clear of drugs.

In one striking passage, she wrote: "Parents, both of you, thank you for teaching me that taking drugs ruins lives, breaks families apart and gives no one a quality of life worth living.

"I'll be eternally grateful for this lesson you have taught me which has a message which has stuck by me until this day and always will, I have never and will never have a desire to take harmful substances through your example."

The letter was published in the same week that Ms Cameron's father, Alexander, was jailed for a series of crimes at Dundee Sheriff Court.

Her mother, Tammy, told the Dundee Evening Telegraph that she was proud of her "amazing daughter".

She said: "No child should have to go through what Chelsea did and live that kind of life.

"I am ashamed and upset at my behaviour and am so sorry and so proud of her."

Ms Cameron said she had a "relatively normal" upbringing but she was aware that heroin and diazepam were a big part of her parents' lives.

She stopped living with them when she was 14, and instead stayed with various family and friends until she got her own home last October.

Despite her problems, the teenager excelled at school and discovered a passion for languages.

She was one of a group of pupils from Menzieshill High School, Dundee, who travelled to Uganda to carry out charity work.

And last year, as head girl, she spoke in front of hundreds of people at the school's prize giving ceremony.

Now she has landed an apprenticeship in administration.

She told the BBC Scotland news website that a teacher once told her class that anyone who was exposed to drug abuse as a child was "absolutely certain" to follow that path.

From that point, she was determined to make a success of her life.

"Society wasn't going to tell me what my future was going to be," she said. "Someone else's choices weren't going to determine my future."

Her positive outlook is reflected in the letter to her unemployed parents, in which she thanks them for teaching her to be ambitious.

She wrote: "Your example showed me that no ambition for education, work or any type of success is very harmful and leads to not a lot of self worth.

"Your example showed me that life is all about choices and that I didn't need to make the same ones you did."

The teenager also reveals that she hid the truth of her family life from her school friends until she was in the third or fourth year of high school.

She added: "Life is not sunshine and rainbows and thank you for teaching me that life is unfair, people disappoint you and there's sometimes nothing you can do about that. A lesson well learnt from the both of you."

She ends the letter: "I hope one day that you'll wake up and realise there is so much more the world has to offer you guys and when that day comes, please come to find me so we can enjoy life together.

"I'll show you some nice restaurants I like to go to and if you're lucky I might take you to Germany one day. Until then, I'll dream of what my life would be like with parents to enjoy it with."