Voller, 53, visited the head of Leverkusen's medical team Dr Karl-Heinrich Dittmar for treatment on Saturday and was taken to hospital immediately as an in-patient, a statement on the club's website explained.
"Bayer 04 wishes Rudi Voller a speedy recovery," it added.
Voller, who scored 47 goals in 90 appearances for West Germany and Germany between 1982 and 1994, missed second-placed Leverkusen's 3-0 home win over Nuremberg on Saturday.
A Son Heung-Min double and Stefan Kiessling's strike earned Leverkusen the victory to ensure Samy Hypia's side stayed within four points of leaders Bayern Munich.
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