Drugs cause hair loss by interfering with the normal cycle of scalp hair growth. During the anagen phase, which lasts for three to four years, the hair grows. During the telogen phase, which lasts about three months, the hair rests. At the end of the telogen phase, the hair falls out and is replaced by a new hair.

Medications can lead to two types of hair loss: telogen effluvium and Anagen effluvium.

Telogen effluvium is the most common form of drug-induced hair loss. It usually appears within two to four months after taking the drug. This condition causes the hair follicles to go into their resting phase (telogen) and fall out too early. People with telogen effluvium usually shed between 100 and 150 hairs a day.

Anagen effluvium is hair loss that occurs during the anagen phase of the hair cycle, when the hairs are actively growing. It prevents the matrix cells, which produce new hairs, from dividing normally. This type of hair loss usually occurs within a few days to weeks after taking the medication. It's most common in people who are taking chemotherapy drugs and is often severe, causing people to lose most or all of the hair on their head, as well as their eyebrows, eyelashes, and other body hairs.

The severity of drug-induced hair loss depends on the type of drug and dosage, as well as your sensitivity to that drug.

Many different types of drugs are thought to cause hair loss, including:                 

·         Acne medications containing vitamin A (retinoids)

·         Antibiotics and antifungal drugs

·         Antidepressants   

·         Birth control pills

·         Anticlotting drugs

·         Cholesterol-lowering drugs  

·         Drugs that suppress the immune system  

·         Hormone replacement therapy  

·         Mood stabilizers