Substance-administration apparatus (20) is provided, including a housing (22), and a needle (80), couplable to the housing (22), having a skin-insertion portion 0.3-2.5 mm in length and an outer diameter less than 0.23 mm. A dosage regulator (60) regulates a dosage of substance injected in a single injection through the needle (80). Dosage is selectable by a setting of the dosage regulator (60) as one of a plurality of selectable dosages. At least one dosage is less than or equal to 10 ul. An injection driver (54), activatable by a user, drives the substance through the needle (80). A motor coupled to the injection driver (54) is actuated by the dosage regulator (60) in a pulsatile manner to produce brief periods of high pressure in the apparatus (20) that expel a predetermined volume of the substance through the needle (80) at a controlled rate. Other applications are also described.