The fluid in the ear canals triggers a sensor that sends signals to the brain relating to balance. Poor circulation in the ear affects this necessarily fine-tuned process, which is itself a delicate balance.
Some medicine, including high-profile antibiotics such as streptomycin and gentamicin, can affect balance. So can some chemotherapy drugs, including cisplatin and carboplatin. Though not everyone is affected by these drugs in the same way, in some cases they can damage the hearing mechanisms of the inner ear.