We are travelling to Thailand this summer but I am susceptible to insects bites. I will use a repellent containing 50% deet, but will it be compromised by the use of cosmetics, moisturisers, sun-tan cream and after-sun lotions?

Dr Jules Eden replies: If you have to use several creams as well as your mossie repellant, then I suggest you apply the latter last of all. Moisturisers obviously need to be worked nicely into your outer layer of skin, the epidermis, in order to work. But deet works best if it is sensed by the bugs, and hence prevents bites by repelling them. Scented creams could reduce this effect if used at the same time.

Your correspondent (May 31) is right to worry about whether amoxicillin might affect his ability to fly. Despite no previous adverse reaction to antibiotics, I vomited for 14 hours non-stop when I was given the drug by a dentist. So I would be worried if the pilot of my plane had taken it.

The nature of allergy is that on the first instance there may be a mild reaction like swelling, itching and redness. The next time it can be fatal, if anaphylaxis occurs. That's the bad news. The good news is that this is unlikely to happen with amoxicillin. First, your pilot would not be taking it a second time if there was a problem before. Second, I'm sure there are rules governing the use of medication and flying.

· If you have any questions you want to ask Dr Jules Eden, email travel.doctor@guardian.co.uk, or write to The Flying Doctor, The Guardian, 119 Farringdon Road, London EC1R 3ER.