The answer is no.
It is illegal to sell drugs or give them away once they have been dispensed to you. Although, you will have taken care to look after them in conditions recommended by the manufacturer, it cannot be proven that they haven’t been tampered with – unless they were kept in the fertility clinic or a hospital environment. Unused fertility medicines have to be handed in to a pharmacy or clinic, where they will be destroyed.
The Department of Health’s Medicines Control Agency states that it is illegal for a patient to sell or supply prescription-only or pharmacy-only medicines for another patient (that covers all fertility medicines). They can only be supplied under the supervision of a pharmacist and, if prescription-only, against a doctor’s prescription.
If the doctor or pharmacist wishes to re-use medicines, the law does not prohibit it. However, under the pharmacist’s code of ethics, pharmacists must not consider reusing any medicine returned by a patient; doctors have similar professional standards. The issue really is one of ‘good practice’ for health professionals; and the answer is always ‘no’ unless the medicines concerned were kept at the clinic at all times, as would happen on a hospital ward where medicines can be re-used by other patients.
No one wants to risk using fertility medicines that have been tampered with, accidentally or deliberately, by the previous patient. The only option you have is to take the drugs back to the pharmacy or company you purchased them from for them to dispose of safely.