Medications may cause hair loss. Some anticoagulants and many chemotherapy regimes used in cancer treatments are well known for this. Hair usually returns once the medication stops. Some anti-depressants may result in hair loss and too much vitamin A can have the same effect.
Other physical reasons for hair loss includes hormonal imbalances, over or under-active thyroid and certain fungal infections can all result in hair loss.
Hair loss may also signal some underlying problem like diabetes or lupus. As it is not always possible to assess the cause for rapid or excessive hair loss it is always worth seeking medical advice to diagnose the cause for your hair loss.