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 €ƒ€ƒ€ƒ€ƒ€ƒ€ƒ€ƒ€ƒ R 2 2 N-CH 2 COCH 2 -CH 2 CO-OR 1 €ƒ€ƒ€ƒ€ƒ€ƒ(I) wherein R 1 represents a substituted or unsubstituted, straight-chained, branched or cyclic alkyl group； and； each R 2 independently represents a hydrogen atom or an optionally substituted alkyl group, e.g. a group R 1 ； 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/cm 2 (e.g. 5 to 40 mW/cm 2 ), (iv) said photoactivation is carried out with sunlight.
