High-purity and low-doped n-type epitaxial layers of 4H-SiC have been implanted with N and C ions by using energies in the MeV range and doses from 2×108 to 1×109 cm-2. Postimplant annealing was performed at 1100 °C prior to sample analysis by deep-level transient spectroscopy (DLTS). A drastic and irreversible instability of the prominent EH7 deep-level defect occurs during the first DLTS temperature scan because of the electric field applied during the measurements. Depending on the implanted species, EH7 can decrease (N implants) as well as increase (C implants) in strength and the effect is attributed to charge-state controlled annealing and formation processes of EH7. The origin of EH7 is discussed and the experimental data support a model invoking interstitial C atoms.