The 23-year-old – who had flown from London to LA for the season two launch party of the hit Netflix show – was caught by sniffer dogs at LAZ airport after officials found traces of the illegal substance on his personal items.
Charlie was then sent back to the UK, resulting in him missing the premiere of the supernatural show.
“He was not arrested. He was held at the airport for a while and then put back on a plane to London that night. If you are a foreign national and it is a large amount of drugs you may be charged,” a source told The Sun.
“But if it is personal use, the consequences are that you will not be allowed in to the country. US law is very strict on this.”
Charlie plays Jonathan Byers, brother of abducted schoolboy Will, in the Netflix series.
It is not known if this incident will cause him further problems entering the US, where Stranger Things is filmed.