Hi!Your headache could come as a result of high blood pressure. Hydrochlorothiazid is one of the first choices for the start of therapy, but first we need to make sure if the values for blood pressure are consistent at these values and this is confirmed by a regular measurement two times a day for one week or by placing a 24 hour blood pressure monitor (Holter blood pressure). After seeing the true values of blood pressure during 24 h we may decide for the therapy. Hydrochlorthiazide can be started as mono-therapy or as combined therapy. Dizziness is a common side effect at the beginning of treatment with hydroclorthiazide as your body adjusts to the medication.Normally it goes away some days after the treatment has started. To reduce the risk of dizziness, get up slowly when rising from a sitting or lying position.If concerns persist then immediately interrupt the treatment with hydrochlorothiazid and consult your GP to start another therapy for HBP.