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In most cases, you can deduct the cost of materials and supplies only to the extent you actually consumed and used them in your business during the tax year (unless you deducted them in a prior tax year). However, if you had incidental materials and supplies on hand for which you kept no inventories or records of use, you can deduct the cost of those you actually purchased during the tax year, provided that method clearly reflects income.

You can also deduct the cost of books, professional instruments, equipment, etc., if you normally use them within a year. However, if their usefulness extends substantially beyond a year, you must generally recover their costs through depreciation.