Pursuant to Patterson v. State, 238 Ga. 204 (232 S.E.2d 233) (1977), an accused charged with possession or sale of a prohibited substance is entitled to have an independent examination of the substance made. That right, however, is not absolute. The demand must be timely, the request must be reasonable, and the trial court may, as a matter of discretion, refuse to permit such an examination. Id. at 206. In Patterson, the demand was not made until nearly four years after Patterson's arrest and shortly before his trial. Two months before the request, the marijuana had been destroyed by the State Crime Laboratory. There, the demand was found untimely, and there was no impediment to prosecution. Even destruction of the entire seized material during testing by the State will not preclude prosecution and admission of the State's test results. In Partain v. State, 238 Ga. 207, 208 (232 S.E.2d 46) (1977), it was concluded that a request for independent testing when the substance had been used up was unreasonable because an independent test was impossible.