A second positive drug netted Allmendinger an indefinite suspension Tuesday. He had previously been placed on temporary suspension following a first positive result on a urine sample collected last month at Kentucky Speedway.
A similar result for his "B" sample put him in violation of NASCAR's substance abuse policy.
NASCAR has a policy of not revealing what drugs are found, but Allmendinger's business manager, Tara Ragan, confirmed he tested positive for amphetamines.
The 30-year-old driver indicated Wednesday he would participate, starting immediately, in a recovery program required for his reinstatement.
Allmendinger promised in a Twitter posting Tuesday to get back on the track.
"I just wanted to say thank you first and foremost for all (of) you sticking by me," he wrote. "I'm sorry we even have to have this going on. But I promise I will do whatever it takes to get to the bottom of this and get back out there...Thanks guys."