Q My sex life is great, but I want it to be even better. Can I ask my doctor for Viagra if I am prepared to pay for it? HW from Cardiff

A No. There are strict prescribing criteria for drugs used to treat erectile dysfunction. This is mainly on grounds of cost, but also because they can have dangerous side effects if given to patients taking nitrate treatment or where sex is inadvisable. Appropriate conditions where they can be prescribed include renal failure, diabetes, spinal-cord injury and radical pelvic surgery.

Q What should I eat to stop my hair from being so dry and brittle? I’ve tried various shampoos and conditioners to no avail. Is there a vitamin or mineral I’m lacking, and could this be a symptom of a more serious problem? LT from Cheshire

A The obvious suspect here is an underactive thyroid gland, also known as myxoedema. This gland, situated in the neck, is critical to the metabolism. If it becomes underactive, symptoms include fatigue, weight gain, low mood, a hoarse voice and dry skin with brittle, lifeless hair. Diagnosis is with a simple blood test. It can take months or years to develop, so sufferers may not realise they have a problem. Get checked for this possibility now.

Q I have developed pain in my left knee when going upstairs and it now feels unreliable. I have had problems with my pelvis being out of alignment for some years, for which I see a chiropractor and do exercises. My GP says I will need an operation, but a physio says I have wasted muscle and should do exercises to strengthen it. Is an operation a good idea? I am 53.

SN via e-mail

A You need to see an orthopaedic knee specialist. A painful, weak and unreliable knee that sometimes gives way suggests derangement of the internal architecture, perhaps caused by an injury. A keyhole examination of your knee will be needed. This may be the operation your GP is thinking of. Any damage found can often be repaired at the same time.

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