1.   A prescription is required because the authorities fear that overuse of the drug would lead to resistant forms of the virus developing.

2.   But a main concern has been the appearance of resistant forms of the bacteria that cause tuberculosis.

3.   But experts said the drug, which requires twice-daily injection, may grant only a brief reprieve for patients with resistant forms of infection.

4.   But his stubborn viral levels in the face of treatment mean the HIV in his system has mutated into a resistant form.

5.   But resistant forms of the disease have been found in virtually every part of the United States.

6.   The rapid turnover of virus now known to take place explains why the resistant forms can so quickly predominate in the body after a drug has been administered.

7.   The rise in drug-resistant strains probably occurred from excessive prescription of antibiotics by doctors, forcing microbes to mutate into resistant forms in order to survive.

8.   And the half-hearted drug treatment has given another batch of TB bacilli the time and space to mutate into resistant forms.

9.   In the United States, researchers have reported an abrupt upswing over the past two years in the prevalence of resistant forms of the virus in newly infected people.

10.   Missing a few can let the virus mutate into forms resistant to the drugs.