Many tons and numerous varieties of pharmaceutical substances are used in the UK and other countries. They enter the environment via excretion, improper disposal or industrial waste. If they are not treated and degraded completely they could contaminate the environment. Little is known about their biodegradability. Department of Health (UK) data have been analysed to identify the prescription pharmaceuticals that are most consumed in the UK. This study has produced a list of the 100 most prescribed pharmaceutical substances by mass, which has not been available until now. Co-codamol (codeine phosphate/paracetamol, 444.34 t), paracetamol (403.11 t), co-proxamol (dextropropoxyphene HCl/paracetamol, 336.21 t), metformin hydrochloride (205.79 t) and ibuprofen (162.91 t) are the top five most consumed pharmaceutical substances in the UK. A comparison of these data with information about the biodegradability of the substances in sewage treatment works points to five substances that may accumulate and lead to future problems in the aquatic environment: oxytetracycline, amitriptyline, thioridazine, fluoxetine and dextroproxyphene. However, there may be other non-biodegradable pharmaceutical substances, which have not been reported so far. It is suggested that environmental risk assessment and methods for the removal of these substances from water need to be developed and evaluated, and that field studies on their fates are required.