If you are talking about Optional subjects for UPSC Mains, Pharamcy is not one of the optional subjects. The optional subjects are:

• Law
• Agriculture
• Animal Husbandry and Veterinary Science
• Botany
• Anthropology
• Chemistry
• Civil Engineering
• Accountancy and Commerce
• Economics
• Electrical Engineering
• Geography
• Indian History
• Geology
• Management
• Mechanical Engineering
• Mathematics
• Medical Science
• Phiolosophy
• Physics
• Political Science and International Relations
• Psychology
• Public Administration
• Sociology
• Statistics
• Zoology

So you have to select one of these subjects that might be closer to Pharmacy.