Thanks for your post and clarification. Chronic pain is not listed here.
People with certain medical conditions can get free NHS prescriptions if:
they have one of the conditions listed below, and
they hold a valid medical exemption certificate
Medical exemption certificates are issued on application to people who have:
a permanent fistula (for example, a caecostomy, colostomy, laryngostomy or ileostomy) requiring continuous surgical dressing or requiring an appliance
a form of hypoadrenalism (for example, Addison's disease) for which specific substitution therapy is essential
diabetes insipidus or other forms of hypopituitarism
diabetes mellitus, except where treatment is by diet alone
hypoparathyroidism
myasthenia gravis
myxoedema (hypothyroidism requiring thyroid hormone replacement)
epilepsy requiring continuous anticonvulsive therapy
a continuing physical disability that means the person can't go out without the help of another person – temporary disabilities don't count, even if they last for several months
They are also issued for people undergoing treatment for cancer:
including the effects of cancer, or
the effects of current or previous cancer treatment