McLaren's Sergio Perez confirmed he had endured a very "early morning" as the result of a surprise urine test.
Fernando Alonso said the testers, ensuring FIA-sanctioned drivers comply with the World Anti-Doping Agency (WADA)'s international standard, arrived at his house at 6.42am.
"Got drug tested early hours this morning," added Australian Daniel Ricciardo.
"All of a sudden p*ssing in your own home becomes difficult with someone staring over your shoulder," he said on Twitter.
The WADA standard includes drug testing at grands prix, but also 'out of competition'.