The sex hormones are responsible for the development and regulation of the reproductive system and the characteristics that distinguish adult men and women. Oestrogens are usually associated with women and androgens are usually associated with men. However, both kinds of sex hormone are vital for men and women.

Hughes Syndrome (antiphospholipid syndrome) is an autoimmune condition which affects the blood and its ability to clot. It can cause a range of possible symptoms, some of which are also experienced by people with multiple sclerosis (MS).

The human leukocyte antigen (HLA) family of genes provide the instructions to make a group of proteins known as the HLA complex, which help the immune system to distinguish between foreign invaders, such as bacteria or viruses, and the body's own tissue. HLA is the human version of the major histocompatability complex (MHC), a gene family which occurs in many species.

Hyperbaric oxygen therapy (HBO or HBOT, also called high dose oxygen therapy (HDOT)) involves breathing oxygen through a mask in a pressurised chamber, similar to a diving bell.

Hypnosis or hypnotherapy belongs to a branch of medicine known as complementary and alternative medicine.

The NHS in England recently presented an algorithm for the treatment options in MS. This describes the criteria where each licenced disease modifying drug (DMD) could be prescribed, depending on the type of MS you have and how it has responded to treatment.

Ibudilast is an experimental drug treatment for secondary and primary progressive multiple sclerosis (MS). It is taken as a tablet twice daily.

Imipramine is a drug used to treat depression. In multiple sclerosis (MS), it is sometimes used to treat pain, particularly nerve pain or altered sensations. It can also be used for some mild bladder symptoms.

The immune system is the body's main defence system against infections or foreign substances. It is involved in nerve damage in MS, and is a key area for research into new MS treatments.