Q I am 17 and have noticed a dramatic change in my behaviour recently: binge drinking, drugs, shoplifting and self-harm. I can go from being happy to feeling depressed for no reason. Is this just a phase, or have I got a problem?

KG from Somerset

A This is not a phase; you have a significant problem with your mental health, possibly bipolar disorder. Your drinking and drug-taking are exacerbating this, and the longer these habits continue, the harder it may be for you to break out of the cycle.

You have had the courage to write to me. Now have the courage to see your doctor and get help such as counselling or medication. If this idea causes you anxiety, call the confidential helpline of Sane, the mental health charity, on 0845 767 8000 for advice.

Q I have a small circular bald patch in my stubble, just below my chin. Why has this appeared?

NW from Southport

A This sounds like a type of alopecia – alopecia barbae – that affects the stubble of men. Although there is often no apparent reason, stress can seem to trigger it and conditions such as diabetes and thyroid-gland imbalances can also be a cause.

See your GP as you may need to be tested for these, and for a fungal infection, which is a possibility. Most cases of alopecia barbae do not progress and gradually resolve with no treatment, although this can take months or even years.

Q I have been diagnosed with plaque psoriasis and would like to know how big the lesions grow. I have some of around 4in by 3in and of an irregular shape, over much of my body.

GH from Leek, Staffordshire

A About 2% of Britons have psoriasis. The exact cause of the skin disorder is unclear but triggers include injury and stress. Plaque psoriasis, the commonest form, consists of red patches or spots that become scaly.

Moisturisers will soften the skin, while steroid creams and coal-tar preparations can also help, along with vitamin D ointments. If severe, consider ultraviolet light therapy. Your plaques are in the normal size range and should not grow bigger, if successfully treated.

All questions are answered by a practising doctor. To consult Dr Know, e-mail drknow@sunday-times.co.uk. E-mails cannot be answered individually