It was, Sam Chalmers admits, the worst mistake of his life.

Two years ago, as a nineteen year-old, Chalmers tested positive for methandienone and stanozolol while representing Scotland under-20s. It is a mistake for which he paid a heavy price; the resultant two-year suspension comes to an end this month, allowing him to resume his rugby career.

Quite apart from this being the first positive test in Scottish rugby history, it was the name of transgressor that made it newsworthy. Sam is the son of Craig Chalmers, the decorated former fly half who won a Grand Slam with Scotland and toured with the British Lions in the 1990s.

Here was a young player aiming to follow in his father's footsteps, but who was derailed by an error of judgment. The 21-year-old is currently in Australia playing for the Sydney club, Southern Districts, ahead of a return to competitive action in mid-June. This is a day that his father, who is preparing to travel Down Under in the coming weeks, cannot wait to see.

"I'm really pleased that he'll be back playing soon - two years is a long time. It's a part of his rugby career that he bitterly regrets but he's done his time now," Chalmers snr told Herald Sport.

"He's done a lot of good work in those two years with doping awareness and he's become more mature. It's been a bad experience but he's reacted positively to it. He's put his hands up to it and it's one of those things that happens sometimes with young people."

Chalmers jnr, who played for Melrose when he tested positive, admitted at the time he had pressure from coaches to bulk up, and this led to him taking a pill called Pro-SD, which contains anabolic androgenic steroids. His father explains that there were also underlying reasons which led to his son taking a banned substance.

"Sam's had a lot to overcome in his life. He's got anaphylaxis, he's got a peanut allergy, he's got bad adenoids, he's diabetic," said the former Scotland No.10.

"He's had to alter his diet because of all of that and when you're diabetic, it's always a bit harder to put weight on so he's had a lot to deal with.

"There were a lot of reasons why he was struggling to put on weight. He wanted to push on and get an opportunity in the national set-up but he was told that he wasn't big enough by some of the coaches there. Size for players in certain positions was an issue - you also need ability but size does seem to be a huge issue."

Chalmers is overwhelmingly supportive of his son but refuses to make excuses for what he did. It is the youngster's reaction to his situation that has really shone through.

"Unfortunately, Sam got in with a couple of the wrong people and made a really bad decision. He made a huge mistake and it cost him dearly - it's cost him two years of his career," said Chalmers.

"He got caught up with these people who I've got no time for at all and I admire how he's reacted to the situation because it was a hellish situation to be in.

"It should never have happened, he should never have gone down that path. I don't think that he'd have thought about it if somebody hadn't shown him what effect taking it had had on them.

"Sam thought it was a quick fix. He only took it for about a week and he had stomach problems because of it - I think it was a reaction to his diabetes and so he stopped taking it but it was too late, the damage was done. It's a period of his life that he'll never forget. He'll learn from his mistake and there's no way that he'll ever make that mistake again."

Chalmers acknowledges that the pressure on his son to succeed in rugby was immense. Any player whose father had been capped 60 times is likely to attract more attention than your average teenager. "I've never put any pressure on my kids but [from outwith the family] there's always expectation and extra pressure and I think that with me being an ex-international player, that made it harder for him," he said.

"I know myself how frustrating it is not to be picked but you just need to dig in and you've got to say, 'I'll show you'. There's lots of pressures there but you just have to be very, very careful about what you do."

In modern-day rugby, players are ritually encouraged to add bulk. As a 19-year-old, Sam was 5ft 11in and weighed just under 13 stone and was continually told he was too small. "I remember a while ago going and getting my British Lions blazer and telling Sam to try it on," recalled Chalmers. "I was the same age as him when I got it and he could hardly get it on. And I told him that when I wore that, it fitted me - I wasn't big at that age either.

"I know that the game is different now, that it's moved on but you've just got to be patient. The young guys need to get good nutritional advice and just be patient."

Sam Chalmers has done a great deal during his two-year suspension to atone for his mistake. He has participated in anti-doping work with the IRB, the game's governing body, including appearing in educational videos in an effort to educate other young players about the dangers of doping both to sporting careers and to health. Talking to younger players about his shame is something that his father knows was not an easy thing to do.

"He's done a lot of good work with the IRB during the Junior World Cup - he did a lot to get his message across," said Chalmers. "He was very emotional at times when he was speaking to other players. Also local athletes, he'd get quite emotional when he was speaking to them.

"He was quite tearful at certain times when he was explaining to parents and young kids what he'd done, how wrong it was and that they shouldn't go down that route. He learnt a lot and he talked about it a lot and I think that's all part of the process of making amends in some small way for what he's done. I fully respect him and admire him for that."

Chalmers is, unsurprisingly, relieved that a dark chapter of his son's life is coming to a close. But it appears that the respect he has for his son has not diminished; rather, it has grown. "He's a strong young man and I'm proud of him in the way that he's handled all of this," he said.

"Yes, I was disappointed briefly but he's my son and I'll always be proud of him but especially how he's dealt with the last two years, how he's taken responsibility for what he's done and faced up to his issues. I'm just glad that it's all coming to an end and I'm looking forward to seeing him do really well."