One of the most important obstacles to combining pharmaceutical agents to treat ocular diseases is the risk of physiochemical reactions. In intraocular administration, these reactions may produce incompatibility, instability, or both. They may change the nature of drug activity, and they may threaten normal cellular function, resulting in lens opacities, corneal toxicity, retinal cell damage, or other adverse outcomes. Although many medications have demonstrated efficacy or have shown promise when administered intravitreally, including antifungals, nonsteroidal antiinflammatory drugs, anti-tumor necrosis factor-alpha agents, mammalian target of rapamycin inhibitors, metalloproteinase inhibitors, antiviral agents, antineoplastic compounds, and antivascular endothelial growth factor therapies, these have been typically tested as single agents. The potential for these agents to be combined will be largely determined by their physiochemical compatibility.