In short: Aspartame is considered safe by the EFSA within the acceptable daily intake (40 mg/kg body weight) — an 85 kg person would need to drink about 7 liters of diet soda per day to exceed that. The WHO classifies it as "possibly carcinogenic," but the evidence is weak with no proven causality. Compared to sugar, it's the better bet.
Sucralose is safe when cold, but becomes problematic above about 120 °C: when heated, it breaks down into carcinogenic chlorinated compounds (dioxins). Germany's Federal Institute for Risk Assessment advises against baking or frying with sucralose. E-cigarettes with sucralose are also affected. In the EU, sucralose is therefore not approved for industrial baked goods.
Bottom line: Drinking aspartame — yes. Heating sucralose — no.
Sources
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EFSA (European Food Safety Authority), Re-evaluation of aspartame (E 951) — full risk assessment, 10. Dezember 2013: https://www.efsa.europa.eu/en/press/news/131210
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WHO/IARC, Aspartame hazard and risk assessment results released, 14. Juli 2023: https://www.iarc.who.int/news-events/aspartame-hazard-and-risk-assessment-results-released/
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IARC Monographs Volume 134, Evaluation of the carcinogenicity of aspartame, methyleugenol, and isoeugenol, 2023: https://monographs.iarc.who.int/iarc-monographs-volume-134/
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BfR (Bundesinstitut für Risikobewertung), Süßstoff Sucralose: Beim Erhitzen von Lebensmitteln können gesundheitsschädliche Verbindungen entstehen: https://www.bfr.bund.de/stellungnahme/suessstoff-sucralose-beim-erhitzen-von-lebensmitteln-koennen-gesundheitsschaedliche-verbindungen-entstehen/
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BfR, Sucralose — Heating above 120 °C can produce substances that are harmful to health (EN): https://www.bfr.bund.de/en/notification/sucralose-heating-above-120c-can-produce-substances-that-are-harmful-to-health/
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BfR, Flavouring substances in e-cigarettes: updated assessment of safrole, sucralose and menthol: https://www.bfr.bund.de/en/notification/flavouring-substances-in-e-cigarettes-updated-assessment-of-safrole-sucralose-and-menthol/
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EFSA, Re-evaluation of sucralose (E 955) as a food additive and evaluation of extension of use in fine bakery wares, EFSA Journal 2026: https://www.efsa.europa.eu/en/efsajournal/pub/9854
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Rahn et al., Polychlorinated dibenzo-p-dioxins and dibenzofurans formed from sucralose at high temperatures, Scientific Reports, 2013: https://www.nature.com/articles/srep02946
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Bornemann et al., Thermal degradation of sucralose: a combination of analytical methods, Scientific Reports, 2015: https://www.nature.com/articles/srep09598
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Erythropel et al., Sucralose-Enhanced Degradation of Electronic Cigarette Liquids during Vaping, Chemical Research in Toxicology, 2019: https://pubmed.ncbi.nlm.nih.gov/31079450/