Guerrero, 33, who plays for Flamengo in Brazil, had been suspended by FIFA on Dec. 8 for a year after testing positive for cocaine metabolite benzoylecgonine after Peru’s 0-0 draw away to Argentina in October.
World soccer's governing body reduced the ban to six months on Wednesday, but Guerrero is still not happy.
"Obviously I am not relieved, it’s important to me to keep (fighting) to prove my innocence," he told Reuters TV on Thursday in Rio de Janeiro.
"I am not calm now because they reduced the ban, I am going to continue fighting to prove my innocence. It really is an injustice that they punished me as I have showed I am innocent all along, I proved it, and we will keep proving it until they absolve me."
"They cut the legs from under me," Guerrero added. "I did absolutely nothing wrong."
Guerrero claimed the positive test was due to contamination and vowed to be back in action in 2018.
The former Bayern Munich and Hamburg SV forward, who has scored 42 goals for Flamengo since moving to Brazil in 2015, gives the Peru attack a much-needed physical presence and has a galvanising effect on their less experienced players.
The 0-0 draw with Argentina was a crucial result for Peru, who qualified for the Russia World Cup for the first time since 1982.
The South American side have been drawn in a group with France, Denmark and Australia.
(Writing by Andrew Downie; Editing by Toby Davis)
