What Factors Affect a Sufficient Ivermectin Dose?

Ivermectin is a semisynthetic antihelmintic, or antiparasitic, agent sold under the brand name Stromectol®. Closely related to other members of the avermectin class of drugs, ivermectin is effective against a broad range of parasites. It has been approved for use in the treatment of onchocerciasis, ascariasis, strongyloidiasis, cutaneous larva migrans, scabies and filariasis. Ivermectin is approved for cases of filariasis in pediatric populations as well, although a lower ivermectin dose should be used. Apart from patient age, other common factors that affect the recommended initial ivermectin dose are diminished liver function, lowered immune system activity, and the type of parasitic condition to be treated.

When treating onchocerciasis, a single oral dose is typically required only once per year, although patients with severe ocular infections may need treatment every three to six months. An ivermectin dose of 3 mg should be used in patients with a body weight between 33 and 55 lbs (15 and 25 kg). Patients weighing between 57.2 and 96.8 lbs (26 and 44 kg) should be given double that dose, while those between 99 and 140.8 lbs (45 and 64 kg) should receive a 9 mg ivermectin dose. Patients between 143 and 184.8 lbs (65 and 84 kg) should be given a 12 mg dose, while heavier patients should be given 0.15 mg per 2.2 lbs (1 kg) of body weight.

In rare cases, some patients with onchocerciasis — both with and without treatment — have developed a potentially fatal brain condition characterized by red eye, back and neck pain, difficulty breathing, loss of urinary and bowel control, bleeding eyes, stupor, loss or coordination, seizures, and comas. Some patients heavily infected with loa loa — commonly referred to as "eye worm" — may experience fatal encephalopathy as well. Since ivermectin alone is not an effective means of eliminating the adult Onchocerca parasites, retreatment or surgical excision may be necessary.

When treating ascariasis, cutaneous larva migrans, scabies or filariasis, a single dose of 0.2 mg per 2.2 lbs (1 kg) of body weight should be sufficient. When treating persistent filariasis infections, an annual dose of 0.4 mg per 2.2 lbs (1 kg) of body weight along with a dose of diethylcarbamazine at 6 mg per 2.2 lbs (1 kg) of weight has proven effective. The treatment of crusted scabies infections may necessitate the administration of two or more ivermectin doses every one or two weeks.

Since the chemical is heavily metabolized in the liver, it is likely that lower doses may be needed in patients with reduced liver function. As of 2011, no studies have been conducted to determine the appropriate ivermectin dose in populations with compromised hepatic function, so this drug should be used with caution in patients with liver disease. Similarly, research on this medicine has not included a large geriatric sample size. Although it is possible that lower dosages may be preferred in patients over the age of 65, the appropriate ivermectin dose has yet to be determined.