Provided are a starting material for a bulk drug and an additive for a drug, said starting material and additive achieving high formulation stability and excellent temporal stability of drug efficacy after being used for modifying a bulk drug, a polypeptide, a physiologically active protein, an enzyme, etc. The present invention pertains to a starting material for a bulk drug or an additive for a drug, said starting material or additive comprising a polyether composition (A) represented by general formula (1), wherein: composition (A) shows a single peak molecular weight distribution; the ratio (Mw/Mn) of the weight-average molecular weight (Mw) of composition (A) to the number-average molecular weight (Mn) thereof is 1.20 or less; and composition (A) contains 90 wt% or more, relative to the weight of composition (A), of a compound wherein m in general formula (1) is 1. In general formula (1): OR1, R2O, R3O and R4O independently represent an oxyalkylene group having 2-8 carbon atoms, provided that multiple OR1's, R2O's, R3O's and R4O's, if present, may be either the same or different and bonded together in either random or block manner; a, b, c and d independently represent an integer of 50-1200; X1 to X4 independently represent a hydrogen atom, a substituent represented by general formula (2) or a substituent represented by general formula (3); and m is an integer of 1-10.
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