The NFL suspended Neblett four games without pay last week.
Neblett said Tuesday he unsuccessfully attempted to appeal the suspension. He said he made a mistake when he took an over-the-counter dietary supplement that resulted in a positive test.
"It's something I know I shouldn't have taken," Neblett said after minicamp practice in Charlotte. "I should have checked with my trainer" first.
The 310-pound Neblett said he took the supplement to keep his weight down.
Neblett, entering his third season with the Panthers, played in 12 games last year with four starts. He finished with 23 tackles and 2.5 sacks. He initially made the roster in 2011 as an undrafted rookie out of Temple.
"This is definitely going to make it harder" to make the roster, Neblett said. "I'm not going to be here for four games, so that gives somebody else a chance to step up and take that spot, just as I did. So it is going to be some work getting back."
Neblett will be allowed to participate in training camp, which begins Saturday in Spartanburg, S.C., and can play in the preseason games. But he won't be allowed to practice with the team for the first four weeks of the regular season. If the Panthers decide to keep him, he won't take up a roster spot the first month of the season, as he can be placed on the reserve-suspended list.
Coach Ron Rivera said the Panthers would treat Neblett "no differently" than anyone else on the roster.
"We had a situation like that in San Diego with a player going through camp and we didn't treat him any differently, either," Rivera said. "What it did was give us a chance to have somebody else up (on the 53-man roster) for a while. Then we had to make a decision afterward."
Rivera said Neblett would be evaluated by his play in training camp and the preseason.
"What Andre did for us last season shows he's a good football player," Rivera said.
Neblett feels like he let his teammates down.
"I'm costing a team that can potentially make it to the playoffs," he said. "My family is disappointed and I'm extremely disappointed."