There are no reported side effects from the long term use of doxycycline, which is easily available from pharmacies in Gambia. Many ex-pats don't take anything, relying on insect repellent, BOP insecticide and mosquito nets. Including garlic and Marmite in your diet is supposed to make you smell nasty to mosquitoes so they avoid you.
If you do get malaria - it's no big deal for adults! The clinics recognise the symptoms and know how to treat malaria. Three injections over 18 hours and that's it.