 - Participant currently required to take or likely to be required to start, in the 6 months following enrolment, a medicinal product which is known or suspected to interact, to a clinically significant extent, with doxycycline including ergotamine and methysergide. This includes drugs known to induce the cytochrome P450 system to a clinically significant extent, including but not limited to, carbamazepine, phenytoin, rifampicin, griseofulvin, barbiturates or sulphonylureas. 