The company, which has consumer brands with a strong presence in overseas markets, is conducting the review out of its Paris head office.
ZenithOptimedia handles the majority of media duties in Europe, but it does not have a significant spend in the UK.
The majority of the company's products are prescription-only, covering seven areas: cardiovascular, central nervous system, diabetes, internal medicine, oncology, thrombosis and vaccines.
Sanofi-Aventis products include the blood pressure treatment Aprovel and the anti-depressant Solain. Its consumer brands include Nature's Own and Bio Organics.
A spokesman for Sanofi-Aventis said: "We are engaging in a review of our media planning and buying arrangements currently handled globally by ZenithOptimedia and are in conversations with other networks regarding the business."
The spokesman added the review did not affect the global creative account, which is split between Publicis and Euro RSCG.
The company is the world's fourth-largest by pharmaceutical sales. It was formed in 2004 in the acquisition of Aventis by the rival pharmaceutical company Sanofi-Synthelabo in a €47.8 billion deal.