The invention provides a composition comprising a photosensitiser which is a derivative of 5-aminolevulinic acid (5-ALA) of formula (I), or a pharmaceutically acceptable salt thereof, which is capable of forming protoporphyrin IX in vivo R22N-CH2COCH2-CH2CO-OR1 (I)whereinR1 represents a substituted or unsubstituted, straight-chained, branched or cyclic alkyl group； and；each R2 independently represents a hydrogen atom or an optionally substituted alkyl group, e.g. a group R1；for use in photodynamic therapy (PDT) on an animal, wherein said PDT comprises:(a) administering to said animal a composition comprising said photosensitiser in a concentration of less than 20 % wt； and(b) photoactivating said photosensitiser,and wherein side-effects of said PDT are prevented or reduced by use of one or more of (ii) to (iv):(ii) said composition is administered for less than 2 hours (e.g. 5 minutes to 90 minutes, preferably 30 minutes to 90 minutes) prior to said photoactivation,(iii) said photoactivation is carried out with a light source having a fluence rate of less than 50 mW/cm2 (e.g. 5 to 40 mW/cm2),(iv) said photoactivation is carried out with sunlight.
