The doctors said they had not come under pressure from authorities while treating Navalny.
Navalny's spokeswoman, Kira Yarmysh, said on Monday supporters had reported what they described as a suspected poisoning to the Russian police and Investigative Committee as soon as Navalny fell ill.
The police and Investigative Committee were not immediately available for comment.
Navalny has been a thorn in the Kremlin's side for more than a decade, exposing what he says is high-level graft and mobilizing crowds of young protesters.
He has been repeatedly detained for organizing public meetings and rallies and sued over his investigations into corruption.
He was barred from running in a presidential election in 2018.
(Writers: Anton Zverev, Andrey Kuzmin, Tom Balmforth | Editor: Andrew Heavens)
Simak breaking news dan berita pilihan kami langsung di ponselmu. Pilih saluran andalanmu akses berita Kompas.com WhatsApp Channel : https://www.whatsapp.com/channel/0029VaFPbedBPzjZrk13HO3D. Pastikan kamu sudah install aplikasi WhatsApp ya.