A replaceable needle assembly is provided for a retractable syringe comprising a barrel and a plunger, whereby the retractable needle can be replaced by a user without affecting the retraction mechanism. A mounting member is removably mountable to the barrel by way of a screw-thread connection and a needle mount is removably coupled to the mounting member. A needle is mounted to the needle mount. The barrel comprises a needle mount retainer that comprises a plurality of fingers that engage the retractable needle mount to prevent inadvertent retraction. The plunger comprises a collapsible seal which maximizes the efficiency of fluid delivery prior to the plunger engaging the retractable needle mount for retraction. An initially compressed spring decompresses to drive retraction of the plunger and the engaged needle mount. A lock formed between the plunger and barrel prevents further use of the plunger after retraction.