The 35-year-old recently took a break from the CBS drama to receive "supervised treatment for prescription pain medication".
"I got hurt pretty bad on the show and I ended up taking painkillers to get to work," O'Loughlin told the Philippine Daily Inquirer. "I had trouble getting off them. That's as complicated as it gets."
He added: "Personally, I'm fantastic [now]. It's like nothing happened."
O'Loughlin, who plays Steve McGarrett, admitted that he had struggled with his temporary absence from Hawaii Five-0.
"It was really difficult for me to be away," he confessed. "The hardest part was... when something is so enormous in your life - I'm at work 14 hours every day - and then all of a sudden, you're not at work."
> Hawaii Five-0: Michelle Borth made series regular for season three
Masi Oka (Max Bergman) previously admitted that it felt strange shooting the show without O'Loughlin.
"It was a different experience, but we definitely missed Alex," he said in April. "A lot of the script had to change, because this was a sudden development for us."
Hawaii Five-0 will return for a third season on CBS in late 2012.
Watch a promo for the Hawaii Five-0 season two finale below: