Antipsychotic medicines are used routinely in people with severe mental illness or learning disability. Antipsychotics often induce hyperprolactinemia (high prolactin level) and in almost all women, and some men, this causes hypogonadism (impaired ovarian or testicular function)often with osteoporosis, partly explaining psychiatric patients' high fracture risk. Reducing prolactin by changing antipsychotic or adding a dopamine agonist often worsens psychosis. Adding aripiprazole to current antipsychotic normalizes prolactin in adult schizophrenic patients, without serious side effects. We thus plan a study of add-on aripiprazole in people with antipsychotic induced hyperprolactinemia.

Our main hypothesis is that aripiprazole will normalize or reduce prolactin sufficiently to restore normal ovarian and testicular function. Our secondary hypothesis is that restoration of normal ovarian and testicular function will improve bone mineral density in patients in whom this was reduced at the time of entry into the study.

Inclusion Criteria:

• Participants willing and able to give informed consent for participation in the study.
• Males or Females, aged 16-50 years (women), 16-60 (men).
• Diagnosed with antipsychotic induced hyperprolactinaemia of sufficient severity to induce secondary hypogonadism.
• Stable dose of current regular antipsychotic medication for at least three months prior to study entry.
• Female participants of child bearing potential willing to ensure that they or their partner use effective contraception during the study and for 1 month thereafter
• Able (in the Investigators opinion) and willing to comply with all study requirements.
• Willing to allow his or her General Practitioner and consultant to be notified of participation in the study.

Exclusion Criteria:

• Pregnancy or breastfeeding
• Any significant disease or disorder which, in the opinion of the Investigator, may either put the participants at risk because of participation in the study, or may influence the result of the study, or the participant's ability to participate in the study.
• Plans to donate blood during the study
• Participants who have participated in another research study involving an investigational product in the past 8 weeks
• Any significant disease or disorder which, in the opinion of the Investigator, may either put the participants at risk because of participation in the study, or may influence the result of the study, or the participant's ability to participate in the study.
• Plans to donate blood during the study
• Participants who have participated in another research study involving an investigational product in the past 8 weeks